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      DOUBLE PRECISION FUNCTION slDTT (UTC)

     - - - -
      D T T
     - - - -

  Increment to be applied to Coordinated Universal Time UTC to give
  Terrestrial Time TT (formerly Ephemeris Time ET)

  (double precision)

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

  Result:  TT-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  Pre 1972 January 1 a fixed value of 10 + ET-TAI is returned.

  3  See also the routine slDT, which roughly estimates ET-UT for
     historical epochs.

  Called:  slDAT

  P.T.Wallace   Starlink   6 December 1994

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

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