### SLA_DAT

TAI$-$UTC

ACTION:
Increment to be applied to Coordinated Universal Time UTC to give International Atomic Time TAI.
CALL:
D = sla_DAT (UTC)
##### GIVEN:
 UTC D UTC date as a modified JD (JD$-$2400000.5)

##### RETURNED:
 sla_DAT D TAI$-$UTC in seconds

NOTES:
(1)
The UTC is specified to be a date rather than a time to indicate that care needs to be taken not to specify an instant which lies within a leap second. Though in most cases UTC can include the fractional part, correct behaviour on the day of a leap second can be guaranteed only up to the end of the second $2{3}^{h}\phantom{\rule{0.3em}{0ex}}5{9}^{m}\phantom{\rule{0.3em}{0ex}}5{9}^{s}$.
(2)
For epochs from 1961 January 1 onwards, the expressions from the file ftp://maia.usno.navy.mil/ser7/tai-utc.dat are used. A 5ms time step at 1961 January 1 is taken from 2.58.1 (p87) of the 1992 Explanatory Supplement.
(3)
UTC began at 1960 January 1.0 (JD 2436934.5) and it is improper to call the routine with an earlier epoch. However, if this is attempted, the TAI$-$UTC expression for the year 1960 is used.
(4)
This routine has to be updated on each occasion that a leap second is announced, and programs using it relinked. Refer to the program source code for information on when the most recent leap second was added.