FUNCTION date2,date1,days

; Returns the date equivalent to date1 + days, where date is
; a date of the form yyyyddd and days is a positive or negative
; number of days.

; B. G. Knapp, 87/02/06

  siz = SIZE(date1)
  IF siz(1) EQ 0 THEN BEGIN
      PRINT,"                                                             "
      PRINT,"    DATE2 returns a double precision date of the form        "
      PRINT,"    YYYYDDD.DD resulting from adding the number of days      "
      PRINT,"    given by its second argument, to the date (also in the   "
      PRINT,"    form YYYYDDD.DD) given by its first argument. The        "
      PRINT,"    calling sequence is illustrated below.                   "
      PRINT,"                                                             "
      PRINT,"    newdate = date2(olddate,ndays)                           "
     RETURN,"                                                             "
  ENDIF

  y = LONG(date1/1000)+9999L
  d = LONG(y*365.25D0)+y/400-y/100+ABS(date1) MOD 1000L+days
  c = LONG(d/36524.25D0)
  d = d+c-LONG(c/4.)
  y = LONG(d/365.250001D0)
  d = ABS(y-9999L)*1000+(d-LONG(y*365.25D0))
  d_typ = SIZE(d)
  IF d_typ(0) EQ 0 THEN BEGIN
     IF y LT 9999 THEN d = -d
  ENDIF ELSE BEGIN
     pick = WHERE(y LT 9999)
     if pick(0) GE 0 then d(pick) = -d
  ENDELSE
  RETURN,d

END