tai_from_utc

lsst.ts.salobj.tai_from_utc(utc: Union[float, str, astropy.time.Time], format: Optional[str] = 'unix') → float

Return TAI in unix seconds, given UTC or any astropy.time.Time.

Smear UTC time out evenly on the day before a leap second, so the day has exactly 86400 UTC seconds (of modified duration). Thus the difference between TAI and UTC is not an integer on those days. See the notes for more information.

Because of the smearing, this function should only be used for scalar measures of UTC, such as unix seconds, Julian Date or Modified Julian Date. It should not be used for ISO-formatted date strings.

Parameters

utcfloat, str or astropy.time.Time

UTC time in the specified format.

formatstr or None

Format of the UTC time, as an astropy.time format name, or None to have astropy guess. Ignored if utc is an instance of astropy.time.Time.

Returns

tai_unixfloat

TAI time in unix seconds.

Raises

ValueError

If the date is earlier than 1972 (which is before integer leap seconds) or within one day of the expiration date of the leap second table (which is automatically updated).

Notes

Smearing During the Day Before a Leap Second

On the day before a leap second this routine follows astropy.time and Standards of Fundamental Astronomy (SOFA): by “smearing” UTC over the whole day, so the day has exactly 86400 seconds of modified duration. This leads to TAI-UTC varying continuously on that day, instead of being an integer number of seconds. See https://github.com/astropy/astropy/issues/10055

The reason for this smearing is to make the function single-valued and continuous near a leap second. This avoids two issues when computing TAI from a scalar value of UTC during a leap second; depending on the sign of the leap second:

• In one case there is a one second period during which there are two possible values of TAI, both equally valid.

• In the other case there is a one second gap in TAI.

To convert UTC to TAI without this smearing: use astropy.time to convert the value, and express the UTC date as an ISO string. ISO format supports 60 <= seconds < 61, whereas scalar representations cannot. Do not use datetime format because neither the datetime library nor astropy.time support datetimes with 60 <= seconds < 61.

On Linux an excellent way to get current TAI on the day of a leap second is to configure ntp or ptp to maintain a leap second table, then use the CLOCK_TAI clock (which is only available on Linux). If you do this then you must configure npt or ptp to make the realtime clock jump at a leap second (rather than smearing over some duration), because on Linux the difference between REALTIME_CLOCK and CLOCK_TAI is always an integer number of seconds.

Updating the Leap Second Table

The leap second table is automatically updated in the background (though updates are very infrequent).