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SUBROUTINE sla_CLYD (IY, IM, ID, NY, ND, JSTAT)
*+
* - - - - -
* C L Y D
* - - - - -
*
* Gregorian calendar to year and day in year (in a Julian calendar
* aligned to the 20th/21st century Gregorian calendar).
*
* Given:
* IY,IM,ID i year, month, day in Gregorian calendar
*
* Returned:
* NY i year (re-aligned Julian calendar)
* ND i day in year (1 = January 1st)
* JSTAT i status:
* 0 = OK
* 1 = bad year (before -4711)
* 2 = bad month
* 3 = bad day (but conversion performed)
*
* Notes:
*
* 1 This routine exists to support the low-precision routines
* sla_EARTH, sla_MOON and sla_ECOR.
*
* 2 Between 1900 March 1 and 2100 February 28 it returns answers
* which are consistent with the ordinary Gregorian calendar.
* Outside this range there will be a discrepancy which increases
* by one day for every non-leap century year.
*
* 3 The essence of the algorithm is first to express the Gregorian
* date as a Julian Day Number and then to convert this back to
* a Julian calendar date, with day-in-year instead of month and
* day. See 12.92-1 and 12.95-1 in the reference.
*
* Reference: Explanatory Supplement to the Astronomical Almanac,
* ed P.K.Seidelmann, University Science Books (1992),
* p604-606.
*
* P.T.Wallace Starlink 26 November 1994
*
* Copyright (C) 1995 Rutherford Appleton Laboratory
*-
IMPLICIT NONE
INTEGER IY,IM,ID,NY,ND,JSTAT
INTEGER I,J,K,L,N
* Month lengths in days
INTEGER MTAB(12)
DATA MTAB/31,28,31,30,31,30,31,31,30,31,30,31/
* Preset status
JSTAT=0
* Validate year
IF (IY.GE.-4711) THEN
* Validate month
IF (IM.GE.1.AND.IM.LE.12) THEN
* Allow for (Gregorian) leap year
IF (MOD(IY,4).EQ.0.AND.
: (MOD(IY,100).NE.0.OR.MOD(IY,400).EQ.0)) THEN
MTAB(2)=29
ELSE
MTAB(2)=28
END IF
* Validate day
IF (ID.LT.1.OR.ID.GT.MTAB(IM)) JSTAT=3
* Perform the conversion
I=(14-IM)/12
K=IY-I
J=(1461*(K+4800))/4+(367*(IM-2+12*I))/12
: -(3*((K+4900)/100))/4+ID-30660
K=(J-1)/1461
L=J-1461*K
N=(L-1)/365-L/1461
J=((80*(L-365*N+30))/2447)/11
I=N+J
ND=59+L-365*I+((4-N)/4)*(1-J)
NY=4*K+I-4716
* Bad month
ELSE
JSTAT=2
END IF
ELSE
* Bad year
JSTAT=1
END IF
END
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