function ymd, longdate

; Given scalar longdate argument, returns array of length 3
; holding the corresponding year, month, and day.

; B. G. Knapp, 86/09/17
;              96.01.31, fix bug: array month_days should hold the
;                        first day of each month, not the last of the
;                        previous month!

  if n_elements(longdate) eq 0 then begin
     print,"                                                            "
     print,"    YMD translates a longdate argument of the form yyyyddd  "
     print,"    into a floating-point vector of length three giving the "
     print,"    corresponding year, month, and day, respectively.  The  "
     print,"    correct calling sequence is illustrated below.          "
     print,"                                                            "
     print,"    y_m_d = YMD(YYYYDDD)                                    "
     return,"                                                           "
  endif
  month_days  = [ [  0, 31, 59, 90,120,151,181,212,243,273,304,334 ], $
                  [  0, 31, 60, 91,121,152,182,213,244,274,305,335 ] ]

  day = longdate MOD 1000
  y = long(longdate/1000)
  leap = ((y mod 4 eq 0) and (y mod 100 ne 0)) or (y mod 400 eq 0)
  p = where(month_days(*,leap)+1 le day)
  m = max(p)+1
  d = day-month_days(m-1,leap)
  return,[y,m,d]
;
  end