; $Id$
;
; Copyright (c) 1988-1998, Research Systems, Inc.  All rights reserved.
;	Unauthorized reproduction prohibited.

function JULDAY, MONTH, DAY, YEAR, Hour, Minute, Second, NDAYSPM = ndayspm, _extra=ex
;+
; NAME:
;	JULDAY
;
; PURPOSE:
;	Calculate the Julian Day Number for a given month, day, and year.
;	This is the inverse of the library function CALDAT.
;	See also caldat, the inverse of this function.
; CATEGORY:
;	Misc.
;
; CALLING SEQUENCE:
;	Result = JULDAY(Month, Day, Year)
;
; INPUTS:
;	MONTH:	Number of the desired month (1 = January, ..., 12 = December).
;
;	DAY:	Number of day of the month.
;
;	YEAR:	Number of the desired year.
;
;	HOUR:	Number of the hour of the day.
;
;	MINUTE:	Number of the minute of the hour.
;
;	SECOND:	
;
; OPTIONAL INPUT PARAMETERS:
;	Hour, Minute, Second = optional time of day.
;
; KEYWORD PARAMETERS:
;
;       NDAYSPM: developpe par eric, ca veut dire: nombre de jours par mois! 
;                par defaut c''est 30, sinon le specifier en donnant
;                une valeur a ndayspm
;                pour passer a un calendrier avec un nombre de jours constant par
;                mois. Utilise en particulier ds julday et caldat
;
; OUTPUTS:
;	JULDAY returns the Julian Day Number (which begins at noon) of the 
;	specified calendar date.  If the time of day (Hr, Min, Sec), is 0,
;	the result will be a long integer, otherwise the result is a 
;	double precision floating point number.
;
; COMMON BLOCKS:
;	None.
;
; SIDE EFFECTS:
;	None.
;
; RESTRICTIONS:
;	Accuracy using IEEE double precision numbers is approximately
;	1/10000th of a second.
;
; MODIFICATION HISTORY:
;	Translated from "Numerical Recipies in C", by William H. Press,
;	Brian P. Flannery, Saul A. Teukolsky, and William T. Vetterling.
;	Cambridge University Press, 1988 (second printing).
;
;	AB, September, 1988
;	DMS, April, 1995, Added time of day.
;
;       Eric Guilyardi, June 1999
;       Added key_work ndayspm for fixed number of days per months
;-
;
   ON_ERROR, 2                  ; Return to caller if errors

   MONTHS = ['JAN','FEB','MAR','APR','MAY','JUN','JUL','AUG', $
             'SEP','OCT','NOV','DEC']

   IF NOT keyword_set(ndayspm)  THEN BEGIN 

; Gregorian Calander was adopted on Oct. 15, 1582
      GREG = 15L + 31L * (10L + 12L * 1582L)

; Process the input, if all are missing, use todays date.
      NP = n_params()
      if NP eq 0 then begin
         DATE = systime()
         L_MONTH = long(where(strupcase(strmid(DATE, 4, 3)) eq MONTHS))
         L_MONTH = L_MONTH[0] + 1 ; Scalarize it...
         L_DAY = long(strmid(DATE, 8, 2))
         L_YEAR = long(strmid(DATE, 20, 4))
      endif else if np ge 3 then begin
         L_MONTH = LONG(MONTH)
         L_DAY = LONG(DAY)
         L_YEAR=LONG(YEAR)
;
;***************************************************
; Change test to allow year 0 for climatological data
;********************************************************
;         if (L_YEAR eq 0) then message, 'There is no year zero.'
      endif else message, 'Wrong number of parameters.'
;
;***************************************************
; Change test to allow year 0 for climatological data
;********************************************************
;      if (L_YEAR lt 0) then L_YEAR = L_YEAR + 1
      if (L_YEAR le 0) then L_YEAR = L_YEAR + 1
      if (L_MONTH gt 2) then begin
         JY = L_YEAR
         JM = L_MONTH + 1
      endif else begin
         JY = L_YEAR - 1
         JM = L_MONTH + 13
      endelse

      JUL = floor(365.25 * JY) + floor(30.6001 * JM) + L_DAY + 1720995
; Test whether to change to Gregorian Calandar.
      if ((L_DAY + 31L * (L_MONTH + 12L * L_YEAR)) ge GREG) then begin
         JA = long(0.01 * JY)
         JUL = JUL + 2 - JA + long(0.25 * JA)
      endif

      if n_elements(Hour) + n_elements(Minute) + n_elements(Second) eq 0 then $
       return, JUL
      if n_elements(Hour) eq 0 then Hour = 0
      if n_elements(Minute) eq 0 then Minute = 0
      if n_elements(Second) eq 0 then Second = 0

      return, JUL + (Hour / 24.0d0 - .5d0) + (Minute/1440.0d0) + (Second / 86400.0d0)

   ENDIF ELSE BEGIN 
;
; Fixed number of days per month (default=30) :
;
      IF ndayspm EQ 1 THEN ndayspm = 30

      L_MONTH = LONG(MONTH)
      L_DAY = LONG(DAY)
      L_YEAR=LONG(YEAR)

      JUL = ((L_YEAR-1)*12. + (L_MONTH-1))* ndayspm + L_DAY 
      if n_elements(Hour) + n_elements(Minute) + n_elements(Second) eq 0 then $
       return, JUL
      if n_elements(Hour) eq 0 then Hour = 0
      if n_elements(Minute) eq 0 then Minute = 0
      if n_elements(Second) eq 0 then Second = 0
      
      return, JUL + (Hour / 24.0d0) + (Minute/1440.0d0) + (Second / 86400.0d0)

   ENDELSE 
end