dat([date] [, /valid])
/valid is not specified, then returns the time
difference â(at) = TAI - UTC in seconds for the given UTC
date, which must have common-calendar year, month, day in
its first dimension but can have more dimensions.
The specified day fraction, if any, is used only for dates before the introduction of leap seconds at the end of 1971.
The returned time difference is for 00:00:00 at the beginning of the calendar day.
Zero is returned for dates before 1960-01-01, when UTC began, and for any case for which a valid result is not available.
The appropriate time difference for future dates is unpredictable, so the current function cannot guarantee accurate results beyond the last date at which its implementation was last updated.
dat(/valid) returns the greatest Julian Date for which this
function claims to provide trustworthy results.
See also: Astronomical Coordinate Calculations