Time and Dates (astropy.time)


The astropy.time package provides functionality for manipulating times and dates. Specific emphasis is placed on supporting time scales (e.g. UTC, TAI, UT1, TDB) and time representations (e.g. JD, MJD, ISO 8601) that are used in astronomy and required to calculate, e.g., sidereal times and barycentric corrections. It uses Cython to wrap the C language ERFA time and calendar routines, using a fast and memory efficient vectorization scheme.

All time manipulations and arithmetic operations are done internally using two 64-bit floats to represent time. Floating point algorithms from [1] are used so that the Time object maintains sub-nanosecond precision over times spanning the age of the universe.

[1]Shewchuk, 1997, Discrete & Computational Geometry 18(3):305-363

Getting Started

The basic way to use astropy.time is to create a Time object by supplying one or more input time values as well as the time format and time scale of those values. The input time(s) can either be a single scalar like "2010-01-01 00:00:00" or a list or a numpy array of values as shown below. In general any output values have the same shape (scalar or array) as the input.

>>> from astropy.time import Time
>>> times = ['1999-01-01T00:00:00.123456789', '2010-01-01T00:00:00']
>>> t = Time(times, format='isot', scale='utc')
>>> t
<Time object: scale='utc' format='isot' value=['1999-01-01T00:00:00.123' '2010-01-01T00:00:00.000']>
>>> t[1]
<Time object: scale='utc' format='isot' value=2010-01-01T00:00:00.000>

The format argument specifies how to interpret the input values, e.g. ISO or JD or Unix time. The scale argument specifies the time scale for the values, e.g. UTC or TT or UT1. The scale argument is optional and defaults to UTC except for Time from epoch formats. We could have written the above as:

>>> t = Time(times, format='isot')

When the format of the input can be unambiguously determined then the format argument is not required, so we can simplify even further:

>>> t = Time(times)

Now let’s get the representation of these times in the JD and MJD formats by requesting the corresponding Time attributes:

>>> t.jd
array([ 2451179.50000143,  2455197.5       ])
>>> t.mjd
array([ 51179.00000143,  55197.        ])

The default representation can be changed by setting the format attribute:

>>> t.format = 'fits'
>>> t
<Time object: scale='utc' format='fits' value=['1999-01-01T00:00:00.123(UTC)'
>>> t.format = 'isot'

We can also convert to a different time scale, for instance from UTC to TT. This uses the same attribute mechanism as above but now returns a new Time object:

>>> t2 = t.tt
>>> t2
<Time object: scale='tt' format='isot' value=['1999-01-01T00:01:04.307' '2010-01-01T00:01:06.184']>
>>> t2.jd
array([ 2451179.5007443 ,  2455197.50076602])

Note that both the ISO (ISOT) and JD representations of t2 are different than for t because they are expressed relative to the TT time scale. Of course, from the numbers or strings one could not tell; one format in which this information is kept is the fits format:

>>> t2.fits
array(['1999-01-01T00:01:04.307(TT)', '2010-01-01T00:01:06.184(TT)'],

Finally, some further examples of what is possible. For details, see the API documentation below.

>>> dt = t[1] - t[0]
>>> dt  
<TimeDelta object: scale='tai' format='jd' value=4018.00002172>

Here, note the conversion of the timescale to TAI. Time differences can only have scales in which one day is always equal to 86400 seconds.

>>> import numpy as np
>>> t[0] + dt * np.linspace(0.,1.,12)
<Time object: scale='utc' format='isot' value=['1999-01-01T00:00:00.123' '2000-01-01T06:32:43.930'
 '2000-12-31T13:05:27.737' '2001-12-31T19:38:11.544'
 '2003-01-01T02:10:55.351' '2004-01-01T08:43:39.158'
 '2004-12-31T15:16:22.965' '2005-12-31T21:49:06.772'
 '2007-01-01T04:21:49.579' '2008-01-01T10:54:33.386'
 '2008-12-31T17:27:17.193' '2010-01-01T00:00:00.000']>
>>> t.sidereal_time('apparent', 'greenwich')
<Longitude [ 6.68050179, 6.70281947] hourangle>

Using astropy.time

Time object basics

In astropy.time a “time” is a single instant of time which is independent of the way the time is represented (the “format”) and the time “scale” which specifies the offset and scaling relation of the unit of time. There is no distinction made between a “date” and a “time” since both concepts (as loosely defined in common usage) are just different representations of a moment in time.

Once a Time object is created it cannot be altered internally. In code lingo it is “immutable.” In particular the common operation of “converting” to a different time scale is always performed by returning a copy of the original Time object which has been converted to the new time scale.

Time Format

The time format specifies how an instant of time is represented. The currently available formats are can be found in the Time.FORMATS dict and are listed in the table below. Each of these formats is implemented as a class that derives from the base TimeFormat class. This class structure can be easily adapted and extended by users for specialized time formats not supplied in astropy.time.

Format Class Example argument
byear TimeBesselianEpoch 1950.0
byear_str TimeBesselianEpochString ‘B1950.0’
cxcsec TimeCxcSec 63072064.184
datetime TimeDatetime datetime(2000, 1, 2, 12, 0, 0)
decimalyear TimeDecimalYear 2000.45
fits TimeFITS ‘2000-01-01T00:00:00.000(TAI)’
gps TimeGPS 630720013.0
iso TimeISO ‘2000-01-01 00:00:00.000’
isot TimeISOT ‘2000-01-01T00:00:00.000’
jd TimeJD 2451544.5
jyear TimeJulianEpoch 2000.0
jyear_str TimeJulianEpochString ‘J2000.0’
mjd TimeMJD 51544.0
plot_date TimePlotDate 730120.0003703703
unix TimeUnix 946684800.0
yday TimeYearDayTime 2000:001:00:00:00.000


The TimeFITS format allows for most but not all of the the FITS standard [2]. Not implemented (yet) is support for a LOCAL timescale. Furthermore, FITS supports some deprecated names for timescales; these are translated to the formal names upon initialization. Furthermore, any specific realization information, such as UT(NIST) is stored only as long as the time scale is not changed.

[2]Rots et al. 2015, A&A 574:A36
Changing format

The default representation can be changed by setting the format attribute in place:

>>> t = Time('2000-01-02')
>>> t.format = 'jd'
>>> t
<Time object: scale='utc' format='jd' value=2451545.5>

Be aware that when changing format, the current output subformat (see section below) may not exist in the new format. In this case the subformat will not be preserved:

>>> t = Time('2000-01-02', format='fits', out_subfmt='longdate')
>>> t.value
>>> t.format = 'iso'
>>> t.out_subfmt
>>> t.format = 'fits'
>>> t.value

The time format classes TimeISO, TimeISOT, TimeFITS, and TimeYearDayTime support the concept of subformats. This allows for variations on the basic theme of a format in both the input string parsing and the output.

The supported subformats are date_hms, date_hm, and date for all but the TimeFITS format; the latter does not support data_hm but does support longdate_hms and longdate for years before the year 0 and after the year 10000. The table below illustrates these subformats for iso, fits, yday formats:

Format Subformat Input / output
iso date_hms 2001-01-02 03:04:05.678
iso date_hm 2001-01-02 03:04
iso date 2001-01-02
fits date_hms 2001-01-02T03:04:05.678(UTC)
fits longdate_hms +02001-01-02T03:04:05.678(UTC)
fits longdate +02001-01-02(UTC)
yday date_hms 2001:032:03:04:05.678
yday date_hm 2001:032:03:04
yday date 2001:032
Time from epoch formats

The formats cxcsec, gps, and unix are a little special in that they provide a floating point representation of the elapsed time in seconds since a particular reference date. These formats have a intrinsic time scale which is used to compute the elapsed seconds since the reference date.

Format Scale Reference date
cxcsec TT 1998-01-01 00:00:00
unix UTC 1970-01-01 00:00:00
gps TAI 1980-01-06 00:00:19

Unlike the other formats which default to UTC, if no scale is provided when initializing a Time object then the above intrinsic scale is used. This is done for computational efficiency.

Time Scale

The time scale (or time standard) is “a specification for measuring time: either the rate at which time passes; or points in time; or both” [3]. See also [4] and [5].

>>> Time.SCALES
('tai', 'tcb', 'tcg', 'tdb', 'tt', 'ut1', 'utc')
Scale Description
tai International Atomic Time (TAI)
tcb Barycentric Coordinate Time (TCB)
tcg Geocentric Coordinate Time (TCG)
tdb Barycentric Dynamical Time (TDB)
tt Terrestrial Time (TT)
ut1 Universal Time (UT1)
utc Coordinated Universal Time (UTC)
[3]Wikipedia time standard article
[4]SOFA Time Scale and Calendar Tools (PDF)

The system of transformation between supported time scales is shown in the figure below. Further details are provided in the Convert time scale section.


Scalar or Array

A Time object can hold either a single time value or an array of time values. The distinction is made entirely by the form of the input time(s). If a Time object holds a single value then any format outputs will be a single scalar value, and likewise for arrays. Like other arrays and lists, Time objects holding arrays are subscriptable, returning scalar or array objects as appropriate:

>>> from astropy.time import Time
>>> t = Time(100.0, format='mjd')
>>> t.jd
>>> t = Time([100.0, 200.0, 300.], format='mjd')
>>> t.jd
array([ 2400100.5,  2400200.5,  2400300.5])
>>> t[:2]
<Time object: scale='utc' format='mjd' value=[ 100.  200.]>
>>> t[2]
<Time object: scale='utc' format='mjd' value=300.0>
>>> t = Time(np.arange(50000., 50003.)[:, np.newaxis],
...          np.arange(0., 1., 0.5), format='mjd')
>>> t
<Time object: scale='utc' format='mjd' value=[[ 50000.   50000.5]
 [ 50001.   50001.5]
 [ 50002.   50002.5]]>
>>> t[0]
<Time object: scale='utc' format='mjd' value=[ 50000.   50000.5]>

Numpy method analogs

For Time instances holding arrays, many of the same methods and attributes that work on ndarray instances can be used. E.g., one can reshape Time instances and take specific parts using reshape(), ravel(), flatten(), T, transpose(), swapaxes(), diagonal(), squeeze(), take():

>>> t.reshape(2, 3)
<Time object: scale='utc' format='mjd' value=[[ 50000.   50000.5  50001. ]
 [ 50001.5  50002.   50002.5]]>
>>> t.T
<Time object: scale='utc' format='mjd' value=[[ 50000.   50001.   50002. ]
 [ 50000.5  50001.5  50002.5]]>

Note that similarly to the ndarray methods, all but flatten() try to use new views of the data, with the data copied only if that it is impossible (as discussed, e.g., in the documentation for numpy reshape()).

Some arithmetic methods are supported as well: min(), max(), ptp(), sort(), argmin(), argmax(), and argsort(). E.g.:

>> t.max()
<Time object: scale='utc' format='mjd' value=50002.5>
>> t.ptp(axis=0)
<TimeDelta object: scale='tai' format='jd' value=[ 2.  2.]>

Inferring input format

The Time class initializer will not accept ambiguous inputs, but it will make automatic inferences in cases where the inputs are unambiguous. This can apply when the times are supplied as datetime objects or strings. In the latter case it is not required to specify the format because the available string formats have no overlap. However, if the format is known in advance the string parsing will be faster if the format is provided.

>>> from datetime import datetime
>>> t = Time(datetime(2010, 1, 2, 1, 2, 3))
>>> t.format
>>> t = Time('2010-01-02 01:02:03')
>>> t.format

Internal representation

The Time object maintains an internal representation of time as a pair of double precision numbers expressing Julian days. The sum of the two numbers is the Julian Date for that time relative to the given time scale. Users requiring no better than microsecond precision over human time scales (~100 years) can safely ignore the internal representation details and skip this section.

This representation is driven by the underlying ERFA C-library implementation. The ERFA routines take care throughout to maintain overall precision of the double pair. The user is free to choose the way in which total JD is provided, though internally one part contains integer days and the other the fraction of the day, as this ensures optimal accuracy for all conversions. The internal JD pair is available via the jd1 and jd2 attributes:

>>> t = Time('2010-01-01 00:00:00', scale='utc')
>>> t.jd1, t.jd2
(2455197.5, 0.0)
>>> t2 = t.tai
>>> t2.jd1, t2.jd2  
(2455197.5, 0.0003935185185185185)

Creating a Time object

The allowed Time arguments to create a time object are listed below:

val : numpy ndarray, list, str, or number
Data to initialize table.
val2 : numpy ndarray, list, str, or number; optional
Data to initialize table.
format : str, optional
Format of input value(s)
scale : str, optional
Time scale of input value(s)
precision : int between 0 and 9 inclusive
Decimal precision when outputting seconds as floating point
in_subfmt : str
Unix glob to select subformats for parsing string input times
out_subfmt : str
Unix glob to select subformats for outputting string times
location : EarthLocation or tuple, optional
If a tuple, 3 Quantity items with length units for geocentric coordinates, or a longitude, latitude, and optional height for geodetic coordinates. Can be a single location, or one for each input time.


The val argument specifies the input time or times and can be a single string or number, or it can be a Python list or numpy array of strings or numbers. To initialize a Time object based on a specified time, it must be present. If val is absent (or None), the Time object will be created for the time corresponding to the instant the object is created.

In most situations one also needs to specify the time scale via the scale argument. The Time class will never guess the time scale, so a simple example would be:

>>> t1 = Time(50100.0, scale='tt', format='mjd')
>>> t2 = Time('2010-01-01 00:00:00', scale='utc')

It is possible to create a new Time object from one or more existing time objects. In this case the format and scale will be inferred from the first object unless explicitly specified.

>>> Time([t1, t2])
<Time object: scale='tt' format='mjd' value=[ 50100.  55197.00076602]>


The val2 argument is available for specialized situations where extremely high precision is required. Recall that the internal representation of time within astropy.time is two double-precision numbers that when summed give the Julian date. If provided the val2 argument is used in combination with val to set the second the internal time values. The exact interpretation of val2 is determined by the input format class. As of this release all string-valued formats ignore val2 and all numeric inputs effectively add the two values in a way that maintains the highest precision. Example:

>>> t = Time(100.0, 0.000001, format='mjd', scale='tt')
>>> t.jd, t.jd1, t.jd2  
(2400100.500001, 2400100.5, 1e-06)


The format argument sets the time time format, and as mentioned it is required unless the format can be unambiguously determined from the input times.


The scale argument sets the time scale and is required except for time formats such as plot_date (TimePlotDate) and unix (TimeUnix). These formats represent the duration in SI seconds since a fixed instant in time which is independent of time scale.


The precision setting affects string formats when outputting a value that includes seconds. It must be an integer between 0 and 9. There is no effect when inputting time values from strings. The default precision is 3. Note that the limit of 9 digits is driven by the way that ERFA handles fractional seconds. In practice this should should not be an issue.

>>> t = Time('B1950.0', scale='utc', precision=3)
>>> t.byear_str
>>> t.precision = 0
>>> t.byear_str


The in_subfmt argument provides a mechanism to select one or more subformat values from the available subformats for string input. Multiple allowed subformats can be selected using Unix-style wildcard characters, in particular * and ?, as documented in the Python fnmatch module.

The default value for in_subfmt is * which matches any available subformat. This allows for convenient input of values with unknown or heterogeneous subformat:

>>> Time(['2000:001', '2000:002:03:04', '2001:003:04:05:06.789'])
<Time object: scale='utc' format='yday'
 value=['2000:001:00:00:00.000' '2000:002:03:04:00.000' '2001:003:04:05:06.789']>

One can explicitly specify in_subfmt in order to strictly require a certain subformat:

>>> t = Time('2000:002:03:04', in_subfmt='date_hm')
>>> t = Time('2000:002', in_subfmt='date_hm')  
Traceback (most recent call last):
ValueError: Input values did not match any of the formats where the
format keyword is optional ['astropy_time', 'datetime',
'byear_str', 'iso', 'isot', 'jyear_str', 'yday']


The out_subfmt argument is similar to in_subfmt except that it applies to output formatting. In the case of multiple matching subformats the first matching subformat is used.

>>> Time('2000-01-01 02:03:04', out_subfmt='date').iso
>>> Time('2000-01-01 02:03:04', out_subfmt='date_hms').iso
'2000-01-01 02:03:04.000'
>>> Time('2000-01-01 02:03:04', out_subfmt='date*').iso
'2000-01-01 02:03:04.000'


This optional parameter specifies the observer location, using an EarthLocation object or a tuple containing any form that can initialize one: either a tuple with geocentric coordinates (X, Y, Z), or a tuple with geodetic coordinates (longitude, latitude, height; with height defaulting to zero). They are used for time scales that are sensitive to observer location (currently, only TDB, which relies on the ERFA routine eraDtdb to determine the time offset between TDB and TT), as well as for sidereal time if no explicit longitude is given.

>>> t = Time('2001-03-22 00:01:44.732327132980', scale='utc',
...          location=('120d', '40d'))
>>> t.sidereal_time('apparent', 'greenwich')  
<Longitude 12.00000000000001 hourangle>
>>> t.sidereal_time('apparent')  
<Longitude 20.00000000000001 hourangle>


In future versions, we hope to add the possibility to add observatory objects and/or names.

Getting the Current Time

The current time can be determined as a Time object using the now class method:

>>> nt = Time.now()
>>> ut = Time(datetime.utcnow(), scale='utc')

The two should be very close to each other.

Using Time objects

There are four basic operations available with Time objects:

  • Get the representation of the time value(s) in a particular time format.
  • Get a new time object for the same time value(s) but referenced to a different time scale.
  • Calculate the sidereal time corresponding to the time value(s).
  • Do time arithmetic involving Time and/or TimeDelta objects.

Get representation

Instants of time can be represented in different ways, for instance as an ISO-format date string ('1999-07-23 04:31:00') or seconds since 1998.0 (49091460.0) or Modified Julian Date (51382.187451574).

The representation of a Time object in a particular format is available by getting the object attribute corresponding to the format name. The list of available format names is in the time format section.

>>> t = Time('2010-01-01 00:00:00', format='iso', scale='utc')
>>> t.jd        # JD representation of time in current scale (UTC)
>>> t.iso       # ISO representation of time in current scale (UTC)
'2010-01-01 00:00:00.000'
>>> t.unix      # seconds since 1970.0 (UTC)
>>> t.plot_date # Date value for plotting with matplotlib plot_date()
>>> t.datetime  # Representation as datetime.datetime object
datetime.datetime(2010, 1, 1, 0, 0)


>>> import matplotlib.pyplot as plt  
>>> jyear = np.linspace(2000, 2001, 20)  
>>> t = Time(jyear, format='jyear')  
>>> plt.plot_date(t.plot_date, jyear)  
>>> plt.gcf().autofmt_xdate()  # orient date labels at a slant  
>>> plt.draw()  

Convert time scale

A new Time object for the same time value(s) but referenced to a new time scale can be created getting the object attribute corresponding to the time scale name. The list of available time scale names is in the time scale section and in the figure below illustrating the network of time scale transformations.



>>> t = Time('2010-01-01 00:00:00', format='iso', scale='utc')
>>> t.tt        # TT scale
<Time object: scale='tt' format='iso' value=2010-01-01 00:01:06.184>
>>> t.tai
<Time object: scale='tai' format='iso' value=2010-01-01 00:00:34.000>

In this process the format and other object attributes like lon, lat, and precision are also propagated to the new object.

As noted in the Time object basics section, a Time object is immutable and the internal time values cannot be altered once the object is created. The process of changing the time scale therefore begins by making a copy of the original object and then converting the internal time values in the copy to the new time scale. The new Time object is returned by the attribute access.

Transformation offsets

Time scale transformations that cross one of the orange circles in the image above require an additional offset time value that is model or observation-dependent. See SOFA Time Scale and Calendar Tools for further details.

The two attributes delta_ut1_utc and delta_tdb_tt provide a way to set these offset times explicitly. These represent the time scale offsets UT1 - UTC and TDB - TT, respectively. As an example:

>>> t = Time('2010-01-01 00:00:00', format='iso', scale='utc')
>>> t.delta_ut1_utc = 0.334  # Explicitly set one part of the transformation
>>> t.ut1.iso    # ISO representation of time in UT1 scale
'2010-01-01 00:00:00.334'

For the UT1 to UTC offset, one has to interpolate in observed values provided by the International Earth Rotation and Reference Systems Service. By default, Astropy is shipped with the final values provided in Bulletin B, which cover the period from 1962 to shortly before an astropy release, and these will be used to compute the offset if the delta_ut1_utc attribute is not set explicitly. For more recent times, one can download an updated version of IERS B or IERS A (which also has predictions), and set delta_ut1_utc as described in get_delta_ut1_utc:

>>> from astropy.utils.iers import IERS_A, IERS_A_URL
>>> iers_a = IERS_A.open(IERS_A_URL)                     
>>> t.delta_ut1_utc = t.get_delta_ut1_utc(iers_a)        

In the case of the TDB to TT offset, most users need only provide the lon and lat values when creating the Time object. If the delta_tdb_tt attribute is not explicitly set then the ERFA C-library routine eraDtdb will be used to compute the TDB to TT offset. Note that if lon and lat are not explicitly initialized, values of 0.0 degrees for both will be used.

The following code replicates an example in the SOFA Time Scale and Calendar Tools document. It does the transform from UTC to all supported time scales (TAI, TCB, TCG, TDB, TT, UT1, UTC). This requires an observer location (here, latitude and longitude).:

>>> import astropy.units as u
>>> t = Time('2006-01-15 21:24:37.5', format='iso', scale='utc',
...          location=(-155.933222*u.deg, 19.48125*u.deg), precision=6)
>>> t.utc.iso
'2006-01-15 21:24:37.500000'
>>> t.ut1.iso
'2006-01-15 21:24:37.834078'
>>> t.tai.iso
'2006-01-15 21:25:10.500000'
>>> t.tt.iso
'2006-01-15 21:25:42.684000'
>>> t.tcg.iso
'2006-01-15 21:25:43.322690'
>>> t.tdb.iso
'2006-01-15 21:25:42.684373'
>>> t.tcb.iso
'2006-01-15 21:25:56.893952'

Sidereal Time

Apparent or mean sidereal time can be calculated using sidereal_time(). The method returns a Longitude with units of hourangle, which by default is for the longitude corresponding to the location with which the Time object is initialized. Like the scale transformations, ERFA C-library routines are used under the hood, which support calculations following different IAU resolutions. Sample usage:

>>> t = Time('2006-01-15 21:24:37.5', scale='utc', location=('120d', '45d'))
>>> t.sidereal_time('mean')  
<Longitude 13.089521870640212 hourangle>
>>> t.sidereal_time('apparent')  
<Longitude 13.089503675087027 hourangle>
>>> t.sidereal_time('apparent', 'greenwich')  
<Longitude 5.089503675087027 hourangle>
>>> t.sidereal_time('apparent', '-90d')  
<Longitude 23.08950367508703 hourangle>
>>> t.sidereal_time('apparent', '-90d', 'IAU1994')  
<Longitude 23.08950365423405 hourangle>

Time Deltas

Simple time arithmetic is supported using the TimeDelta class. The following operations are available:

  • Create a TimeDelta explicitly by instantiating a class object
  • Create a TimeDelta by subtracting two Times
  • Add a TimeDelta to a Time object to get a new Time
  • Subtract a TimeDelta from a Time object to get a new Time
  • Add two TimeDelta objects to get a new TimeDelta
  • Negate a TimeDelta or take its absolute value
  • Multiply or divide a TimeDelta by a constant or array
  • Convert TimeDelta objects to and from time-like Quantities

The TimeDelta class is derived from the Time class and shares many of its properties. One difference is that the time scale has to be one for which one day is exactly 86400 seconds. Hence, the scale cannot be UTC.

The available time formats are:

Format Class
sec TimeDeltaSec
jd TimeDeltaJD


Use of the TimeDelta object is easily illustrated in the few examples below:

>>> t1 = Time('2010-01-01 00:00:00')
>>> t2 = Time('2010-02-01 00:00:00')
>>> dt = t2 - t1  # Difference between two Times
>>> dt
<TimeDelta object: scale='tai' format='jd' value=31.0>
>>> dt.sec

>>> from astropy.time import TimeDelta
>>> dt2 = TimeDelta(50.0, format='sec')
>>> t3 = t2 + dt2  # Add a TimeDelta to a Time
>>> t3.iso
'2010-02-01 00:00:50.000'

>>> t2 - dt2  # Subtract a TimeDelta from a Time
<Time object: scale='utc' format='iso' value=2010-01-31 23:59:10.000>

>>> dt + dt2
<TimeDelta object: scale='tai' format='jd' value=31.0005787037>

>>> import numpy as np
>>> t1 + dt * np.linspace(0, 1, 5)
<Time object: scale='utc' format='iso' value=['2010-01-01 00:00:00.000'
'2010-01-08 18:00:00.000' '2010-01-16 12:00:00.000' '2010-01-24 06:00:00.000'
'2010-02-01 00:00:00.000']>

Time Scales for Time Deltas

Above, one sees that the difference between two UTC times is a TimeDelta with a scale of TAI. This is because a UTC time difference cannot be uniquely defined unless one knows the two times that were differenced (because of leap seconds, a day does not always have 86400 seconds). For all other time scales, the TimeDelta inherits the scale of the first Time object:

>>> t1 = Time('2010-01-01 00:00:00', scale='tcg')
>>> t2 = Time('2011-01-01 00:00:00', scale='tcg')
>>> dt = t2 - t1
>>> dt
<TimeDelta object: scale='tcg' format='jd' value=365.0>

When TimeDelta objects are added or subtracted from Time objects, scales are converted appropriately, with the final scale being that of the Time object:

>>> t2 + dt
<Time object: scale='tcg' format='iso' value=2012-01-01 00:00:00.000>
>>> t2.tai
<Time object: scale='tai' format='iso' value=2010-12-31 23:59:27.068>
>>> t2.tai + dt
<Time object: scale='tai' format='iso' value=2011-12-31 23:59:27.046>

TimeDelta objects can be converted only to objects with compatible scales, i.e., scales for which it is not necessary to know the times that were differenced:

>>> dt.tt  
<TimeDelta object: scale='tt' format='jd' value=364.999999746>
>>> dt.tdb
Traceback (most recent call last):
ScaleValueError: Cannot convert TimeDelta with scale 'tcg' to scale 'tdb'

TimeDelta objects can also have an undefined scale, in which case it is assumed that there scale matches that of the other Time or TimeDelta object (or is TAI in case of a UTC time):

>>> t2.tai + TimeDelta(365., format='jd', scale=None)
<Time object: scale='tai' format='iso' value=2011-12-31 23:59:27.068>

Interaction with Time-like Quantities

Where possible, Quantity objects with units of time are treated as TimeDelta objects with undefined scale (though necessarily with lower precision). They can also be used as input in constructing Time and TimeDelta objects, and TimeDelta objects can be converted to Quantity objects of arbitrary units of time. Usage is most easily illustrated by examples:

>>> import astropy.units as u
>>> Time(10.*u.yr, format='gps')   # time-valued quantities can be used for
...                                # for formats requiring a time offset
<Time object: scale='tai' format='gps' value=315576000.0>
>>> Time(10.*u.yr, 1.*u.s, format='gps')
<Time object: scale='tai' format='gps' value=315576001.0>
>>> Time(2000.*u.yr, scale='utc', format='jyear')
<Time object: scale='utc' format='jyear' value=2000.0>
>>> Time(2000.*u.yr, scale='utc', format='byear')
...                                # but not for Besselian year, which implies
...                                # a different time scale
Traceback (most recent call last):
ValueError: Input values did not match the format class byear

>>> TimeDelta(10.*u.yr)            # With a quantity, no format is required
<TimeDelta object: scale='None' format='jd' value=3652.5>

>>> dt = TimeDelta([10., 20., 30.], format='jd')
>>> dt.to(u.hr)                    # can convert TimeDelta to a quantity
<Quantity [ 240., 480., 720.] h>
>>> dt > 400. * u.hr               # and compare to quantities with units of time
array([False,  True,  True], dtype=bool)
>>> dt + 1.*u.hr                   # can also add/subtract such quantities
<TimeDelta object: scale='None' format='jd' value=[ 10.04166667  20.04166667  30.04166667]>
>>> Time(50000., format='mjd', scale='utc') + 1.*u.hr  
<Time object: scale='utc' format='mjd' value=50000.0416667>
>>> dt * 10.*u.km/u.s              # for multiplication and division with a
...                                # Quantity, TimeDelta is converted
<Quantity [ 100., 200., 300.] d km / s>
>>> dt * 10.*u.Unit(1)             # unless the Quantity is dimensionless
<TimeDelta object: scale='None' format='jd' value=[ 100.  200.  300.]>

Writing a Custom Format

Some applications may need a custom Time format, and this capability is available by making a new subclass of the TimeFormat class. When such a subclass is defined in your code then the format class and corresponding name is automatically registered in the set of available time formats.

The key elements of a new format class are illustrated by examining the code for the jd format (which is one of the simplest):

class TimeJD(TimeFormat):
    Julian Date time format.
    name = 'jd'  # Unique format name

    def set_jds(self, val1, val2):
        Set the internal jd1 and jd2 values from the input val1, val2.
        The input values are expected to conform to this format, as
        validated by self._check_val_type(val1, val2) during __init__.
        self._check_scale(self._scale)  # Validate scale.
        self.jd1, self.jd2 = day_frac(val1, val2)

    def value(self):
        Return format ``value`` property from internal jd1, jd2
        return self.jd1 + self.jd2

As mentioned above, the _check_val_type(self, val1, val2) method may need to be overridden to validate the inputs as conforming to the format specification. By default this checks for valid float, float array, or Quantity inputs. In contrast the iso format class ensures the inputs meet the ISO format spec for strings.

One special case that is relatively common and easier to implement is a format that represents the time since a particular epoch. The classic example is Unix time which is the number of seconds since 1970-01-01 00:00:00 UTC, not counting leap seconds. What if we wanted that value but do want to count leap seconds. This would be done by using the TAI scale instead of the UTC scale. In this case we inherit from the TimeFromEpoch class and define a few class attributes:

>>> from astropy.time.formats import erfa, TimeFromEpoch
>>> class TimeUnixLeap(TimeFromEpoch):
...    """
...    Seconds from 1970-01-01 00:00:00 TAI.  Similar to Unix time
...    but this includes leap seconds.
...    """
...    name = 'unix_leap'
...    unit = 1.0 / erfa.DAYSEC  # in days (1 day == 86400 seconds)
...    epoch_val = '1970-01-01 00:00:00'
...    epoch_val2 = None
...    epoch_scale = 'tai'  # Scale for epoch_val class attribute
...    epoch_format = 'iso'  # Format for epoch_val class attribute

>>> t = Time('2000-01-01')
>>> t.unix_leap
>>> t.unix_leap - t.unix

Going beyond this will probably require looking at the astropy code for more guidance, but if you get stuck the astropy developers are more than happy to help. If you write a format class that is widely useful then we might want to include it in the core!


When a Time object is constructed from a timezone-aware datetime, no timezone information is saved in the Time object. However, Time objects can be converted to timezone-aware datetime objects:

>>> from datetime import datetime
>>> from astropy.time import Time, TimezoneInfo
>>> import astropy.units as u
>>> utc_plus_one_hour = TimezoneInfo(utc_offset=1*u.hour)
>>> dt_aware = datetime(2000, 1, 1, 0, 0, 0, tzinfo=utc_plus_one_hour)
>>> t = Time(dt_aware)  # Loses timezone info, converts to UTC
>>> print(t)            # will return UTC
1999-12-31 23:00:00
>>> print(t.to_datetime(timezone=utc_plus_one_hour)) # to timezone-aware datetime
2000-01-01 00:00:00+01:00

Timezone database packages, like pytz for example, may be more convenient to use to create tzinfo objects used to specify timezones rather than the TimezoneInfo object.


astropy.time Package


OperandTypeError(left, right[, op])
Time(val[, val2, format, scale, precision, ...]) Represent and manipulate times and dates for astronomy.
TimeBesselianEpoch(val1, val2, scale, ...[, ...]) Besselian Epoch year as floating point value(s) like 1950.0
TimeBesselianEpochString(val1, val2, scale, ...) Besselian Epoch year as string value(s) like ‘B1950.0’
TimeCxcSec(val1, val2, scale, precision, ...) Chandra X-ray Center seconds from 1998-01-01 00:00:00 TT.
TimeDatetime(val1, val2, scale, precision, ...) Represent date as Python standard library datetime object
TimeDecimalYear(val1, val2, scale, ...[, ...]) Time as a decimal year, with integer values corresponding to midnight of the first day of each year.
TimeDelta(val[, val2, format, scale, copy]) Represent the time difference between two times.
TimeDeltaFormat(val1, val2, scale, ...[, ...]) Base class for time delta representations
TimeDeltaJD(val1, val2, scale, precision, ...) Time delta in Julian days (86400 SI seconds)
TimeDeltaSec(val1, val2, scale, precision, ...) Time delta in SI seconds
TimeEpochDate(val1, val2, scale, precision, ...) Base class for support floating point Besselian and Julian epoch dates
TimeEpochDateString(val1, val2, scale, ...) Base class to support string Besselian and Julian epoch dates such as ‘B1950.0’ or ‘J2000.0’ respectively.
TimeFITS(val1, val2, scale, precision, ...) FITS format: “[±Y]YYYY-MM-DD[THH:MM:SS[.sss]][(SCALE[(REALIZATION)])]”.
TimeFormat(val1, val2, scale, precision, ...) Base class for time representations.
TimeFromEpoch(val1, val2, scale, precision, ...) Base class for times that represent the interval from a particular epoch as a floating point multiple of a unit time interval (e.g.
TimeGPS(val1, val2, scale, precision, ...[, ...]) GPS time: seconds from 1980-01-06 00:00:00 UTC For example, 630720013.0 is midnight on January 1, 2000.
TimeISO(val1, val2, scale, precision, ...[, ...]) ISO 8601 compliant date-time format “YYYY-MM-DD HH:MM:SS.sss...”.
TimeISOT(val1, val2, scale, precision, ...) ISO 8601 compliant date-time format “YYYY-MM-DDTHH:MM:SS.sss...”.
TimeJD(val1, val2, scale, precision, ...[, ...]) Julian Date time format.
TimeJulianEpoch(val1, val2, scale, ...[, ...]) Julian Epoch year as floating point value(s) like 2000.0
TimeJulianEpochString(val1, val2, scale, ...) Julian Epoch year as string value(s) like ‘J2000.0’
TimeMJD(val1, val2, scale, precision, ...[, ...]) Modified Julian Date time format.
TimePlotDate(val1, val2, scale, precision, ...) Matplotlib plot_date input:
TimeString(val1, val2, scale, precision, ...) Base class for string-like time representations.
TimeUnique(val1, val2, scale, precision, ...) Base class for time formats that can uniquely create a time object without requiring an explicit format specifier.
TimeUnix(val1, val2, scale, precision, ...) Unix time: seconds from 1970-01-01 00:00:00 UTC.
TimeYearDayTime(val1, val2, scale, ...[, ...]) Year, day-of-year and time as “YYYY:DOY:HH:MM:SS.sss...”.
TimezoneInfo([utc_offset, dst, tzname]) Subclass of the tzinfo object, used in the to_datetime method to specify timezones.

Class Inheritance Diagram

Inheritance diagram of astropy.time.core.OperandTypeError, astropy.time.core.ScaleValueError, astropy.time.core.Time, astropy.time.formats.TimeBesselianEpoch, astropy.time.formats.TimeBesselianEpochString, astropy.time.formats.TimeCxcSec, astropy.time.formats.TimeDatetime, astropy.time.formats.TimeDecimalYear, astropy.time.core.TimeDelta, astropy.time.formats.TimeDeltaFormat, astropy.time.formats.TimeDeltaJD, astropy.time.formats.TimeDeltaSec, astropy.time.formats.TimeEpochDate, astropy.time.formats.TimeEpochDateString, astropy.time.formats.TimeFITS, astropy.time.formats.TimeFormat, astropy.time.formats.TimeFromEpoch, astropy.time.formats.TimeGPS, astropy.time.formats.TimeISO, astropy.time.formats.TimeISOT, astropy.time.formats.TimeJD, astropy.time.formats.TimeJulianEpoch, astropy.time.formats.TimeJulianEpochString, astropy.time.formats.TimeMJD, astropy.time.formats.TimePlotDate, astropy.time.formats.TimeString, astropy.time.formats.TimeUnique, astropy.time.formats.TimeUnix, astropy.time.formats.TimeYearDayTime, astropy.time.formats.TimezoneInfo

Acknowledgments and Licenses

This package makes use of the ERFA Software ANSI C library. The copyright of the ERFA software belongs to the NumFOCUS Foundation. The library is made available under the terms of the “BSD-three clauses” license.

The ERFA library is derived, with permission, from the International Astronomical Union’s “Standards of Fundamental Astronomy” library, available from http://www.iausofa.org.