[SubRT] Converts Gregorian dates to Julian days

PRO G2JDAY, YR, MN, DAY, HR, JULIAN

; NAME: G2JDAY
; PURPOSE: Converts Gregorian dates to Julian days
; CALLING SEQUENCE: JDCNV, YR, MN, DAY, HR, JULIAN
;
; INPUTS: YR = Year (integer); MN = Month (integer 1-12); DAY = Day (integer 1-31);
; HR = Hours and fractions of hours of universal time (U.T.)
; OUTPUTS: JULIAN = Julian date (double precision)
; EXAMPLE: To find the Julian Date at 1978 January 1, 0h (U.T.)
; IDL> JDCNV, 1978, 1, 1, 0., JULIAN
; will give JULIAN = 2443509.5
; NOTES:
; REVISON HISTORY:; Converted to IDL from Don Yeomans Comet Ephemeris Generator,
; B. Pfarr, STX, 6/15/88; Converted to IDL V5.0 W. Landsman September 1997

On_error,2
if N_params() LT 5 then begin
print,'Syntax - JDCNV, yr, mn, day, hr, julian
print,' yr - Input Year (e.g. 1978), scalar or vector
print,' mn - Input Month (1-12), scalar or vector
print,' day - Input Day (1-31), scalar or vector
print,' hr - Input Hour (0-24), scalar or vector
print,' julian - output Julian date'
return
endif
yr = long(yr) & mn = long(mn) & day = long(day) ;Make sure integral
L = (mn-14)/12 ;In leap years, -1 for Jan, Feb, else 0
julian = day - 32075l + 1461l*(yr+4800l+L)/4 + $
367l*(mn - 2-L*12)/12 - 3*((yr+4900l+L)/100)/4
julian = double(julian) + (HR/24.0D) - 0.5D
return
end