Procedure

     GR2JUL (Gregorian to Julian Calendar)

     ENTRY GR2JUL (  YEAR, MONTH, DAY, DOY )

Abstract

     Convert Year Month and Day on the  Gregorian Calendar
     to the Julian Calendar

Required_Reading

     None.

Keywords

     TIME

Declarations

    INTEGER               YEAR
    INTEGER               MONTH
    INTEGER               DAY
    INTEGER               DOY

Brief_I/O

     VARIABLE  I/O  DESCRIPTION
     --------  ---  --------------------------------------------------
     YEAR      I-O  Year  of Gregorian Calendar/Julian Calendar
     MONTH     I-O  Month of Gregorian Calendar/Julian Calendar
     DAY       I-O  Day of Month in Gregorian Calendar/Julian Calendar
     DOY        O   Day of Year in Julian Calendar

Detailed_Input

     YEAR     is an integer representing the year of an epoch, E, in
              the Gregorian calendar. Note that the year 0 (zero)
              and negative values are required to represent
              years in the pre-Christian era (B.C.) A year, Y B.C.
              should be represented as -(Y-1).  For example the year
              435 B.C. should be input as -434.

     MONTH    is an integer representing the month of some epoch, E,
              in the Gregorian calendar. Months
              outside the usual range from 1 to 12 are converted
              to the standard range using modular arithmetic and
              the input year is adjusted appropriately.

     DAY      is the day of the month of some epoch, E, in the
              Gregorian calendar.

              Note to input an epoch as the day of a year, set MONTH
              to 1 and DAY to the day of the year.

Detailed_Output

     YEAR     is an integer representing the year of the epoch, E,
              above in the Julian calendar. Note that the year 0
              (zero) and negative values are used to represent
              years in the pre-Christian era (B.C.) A year, Y B.C.,
              is be represented as -(Y-1).  For example the year
              435 B.C. will be returned as -434.

     MONTH    is an integer representing the month of the epoch, E,
              above in the Julian Calendar calendar.

     DAY      is the day of the month of the epoch, E, above in the
              Julian Calendar

     DOY      is the day of the year of the epoch, E, above in the
              Julian Calendar.

Parameters

     None.

Exceptions

     None.

Files

     None.

Particulars

     This is a mid-level utility routine to assist in the assignment
     and presentation of Ancient epochs.

     The SPICE software deals primarily with epochs represented on
     in the Gregorian Calendar. However, the Gregorian calendar
     was not adopted until October 15, 1582. As a result, epochs
     prior to that time are usually represented in the Julian
     proleptic calendar.

     Formally, both calendars can be extended indefinitely forward
     and backward in time due the algorithmic nature of the
     determination of calendar representation.

     This routine allows you to take a numeric representation of
     an epoch represented in the Gregorian calendar and
     convert that to an epoch in the Julian calendar.

     To convert from Julian Calendar to Gregorian
     calendar, use the entry point JUL2GR.

Examples

     Suppose you need to print an epoch (given in seconds past the
     J2000 epoch) of some ancient epoch that occurred during
     pre-Christian era, and that you want to represent this epoch
     using the Julian proleptic calendar.

     Here's the declarations we'll need

        INTEGER               YEAR
        INTEGER               MONTH
        INTEGER               DAY
        INTEGER               DOY

        DOUBLE PRECISION      TVEC ( 6 )
        DOUBLE PRECISION      TDB

     You first need to convert TDB (the epoch in Seconds past J2000)
     to a calendar representation.

        TVEC(1) = TDB.

        CALL TTRANS ( 'TDB', 'YMD', TVEC )

     The output time vector will be relative to the Gregorian
     Calendar. Collect the year, month and day from the time
     vector.

        YEAR    = INT ( TVEC(1) )
        MONTH   = INT ( TVEC(2) )
        DAY     = INT ( TVEC(3) )

     The hours, minutes and seconds appear in components 4 through 6
     of the time vector. We can ignore them in the conversion
     of the calendar from Gregorian to Julian.

        CALL GR2JUL ( YEAR, MONTH, DAY, DOY )

     Now create a string from the YEAR, MONTH, DAY and TVEC(4)
     through TVEC(6).

Restrictions

     None.

Literature_References

     None.

Author_and_Institution

     J. Diaz del Rio    (ODC Space)
     W.L. Taber         (JPL)

Version

    SPICELIB Version 1.1.1, 02-OCT-2021 (JDR)

        Reordered header sections. Edited the header to comply with
        NAIF standard.

    SPICELIB Version 1.1.0, 23-FEB-1998 (WLT)

        The routine was upgraded so that it will handle without
        error months that are outside the range from 1 to 12.

    SPICELIB Version 1.0.0, 13-MAR-1996 (WLT)