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    Procedure to perform conversion of dates to one of three possible formats.


    The following date formats are allowed
      format 1: real*8 scalar encoded as:
              year*1000 + day + hour/24. + min/24./60 + sec/24./60/60
              where day is the day of year (1 to 366)
      format 2: Vector encoded as:
              date[0] = year (eg. 2005)
              date[1] = day of year (1 to 366)
              date[2] = hour
              date[3] = minute
              date[4] = second
              To indicate a date only, set a negative hour.
      format 3: string (ascii text) encoded as
              DD-MON-YEAR HH:MM:SS.SS
              (eg. 14-JUL-2005 15:25:44.23)
              YYYY-MM-DD HH:MM:SS.SS (ISO standard)
              (eg. 1987-07-14 15:25:44.23 or 1987-07-14T15:25:44.23)
              DD/MM/YY (pre-2000 option for FITS DATE keywords)
            Time of day segment is optional in all of these.
      format 4: three element vector giving spacecraft time words
      from a Hubble Space Telescope (HST) telemetry packet. Based on
      total number of secs since midnight, JAN. 1, 1979
      format 5: Julian day. As this is also a scalar, like format 1,
      the distinction between the two on input is made based on their
      value. Numbers > 2300000 are interpreted as Julian days.

Calling Sequence

      results = DATE_CONV( DATE, TYPE )


      DATE - input date in one of the possible formats. Must be scalar.
      TYPE - type of output format desired. If not supplied then
              format 3 (real*8 scalar) is used.
                      valid values:
                      'REAL' - format 1
                      'VECTOR' - format 2
                      'STRING' - format 3
                      'FITS' - YYYY-MM-DDTHH:MM:SS.SS'
                      'JULIAN' - Julian date
                      'MODIFIED' - Modified Julian date (JD-2400000.5)
              TYPE can be abbreviated to the single character strings 'R',
              'V', 'S', 'F', 'J', and 'M'.
              Nobody wants to convert TO spacecraft time (I hope!)


      The converted date is returned as the function value.
      Output is -1 if date is unrecognisable.
      If the time of day is omitted from the input, it will also
      be omitted from any output string (format STRING or FITS).
      Note that date-only strings are allowed by the FITS standard.
      For other output formats any missing time of day is set to
        BAD_DATE set to 1B if date is unrecognisable


      IDL> print,date_conv('2006-03-13 19:58:00.00'),f='(f15.5)'
      IDL> print,date_conv( 2006072.8319444d,'F')
      IDL> print,date_conv( 2006072.8319444d,'V')
            2006.00 72.0000 19.0000 57.0000 59.9962
      IDL> print,date_conv( 2006072.8319444d,'J'), f='(f15.5)'


      version 1 D. Lindler July, 1987
      adapted for IDL version 2 J. Isensee May, 1990
      Made year 2000 compliant; allow ISO format input jls/acc Oct 1998
      DJL/ACC Jan 1998, Modified to work with dates such as 6-JAN-1996 where
              day of month has only one digit.
      DJL, Nov. 2000, Added input/output format YYYY-MM-DDTHH:MM:SS.SS
      Replace spaces with '0' in output FITS format W.Landsman April 2006
      Added Julian date capabilities on input and output. M.Perrin, July 2007
      Removed spurious /WARN keyword to MESSAGE W.L. Feb 2012
      ...and another /WARN; added BAD_DATE, drop spurious time-of-day
      output from strings. J. P. Leahy July 2013
      changed all /CONTINUE warning messages to /INFO: can be suppressed
      by setting !QUIET = 1. J. P. Leahy July 2013

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