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      DOUBLE PRECISION FUNCTION slDAT (UTC)

     - - - -
      D A T
     - - - -

  Increment to be applied to Coordinated Universal Time UTC to give
  International Atomic Time TAI (double precision)

  Given:
     UTC      d      UTC date as a modified JD (JD-2400000.5)

  Result:  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 only be guaranteed up to the end of the second 23:59:59.

  2  For epochs from 1961 January 1 onwards, the expressions from the
     file ftp://maia.usno.navy.mil/ser7/tai-utc.dat are used.

  3  The 5ms timestep at 1961 January 1 is taken from 2.58.1 (p87) of
     the 1992 Explanatory Supplement.

  4  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.


     :-----------------------------------------:
     :                                         :
     :                IMPORTANT                :
     :                                         :
     :  This routine must be updated on each   :
     :     occasion that a leap second is      :
     :                announced                :
     :                                         :
     :  Latest leap second:  1999 January 1    :
     :                                         :
     :-----------------------------------------:

  P.T.Wallace   Starlink   31 May 1999

  Copyright (C) 1999 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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