#  Calculate the Julian day number for the specified date.
#  Based on algorithms published in Computer Language magazine,
#  December, 1990.  Used with permission.

proc julian(month, day, year)
   # The Julian day number is the number of days between noon GMT
   # Jan 1, 4713 BC and that date.  (Years B.C. are negative.)
   local a, b, year_corr, Month, i
   set Month = January February March April May June ^
      July August September October November December
   if (year == '') then
      local dt
      set dt = `dt`
      @ year = $dt:3
      if (day == '') then
         @ day = month == '' ? $dt:2 : month
         @ month = $dt:1
      end
   end
   if (month !~ '[0-9]*') then
      @ month = "$lower(month)*"
      for i = 0 to 11 do
         if (lower(Month[i]) =~ month) break
      end
      @ month = i + 1
   end
   if (month < 1 || month > 12 || year < -4713 || year > 2900) then
      echo -2 julian: Invalid Date
      return -1
   end
   # Correct for negative year.
   if (year < 0) then
      @ year++
      @ year_corr = 1
      @ b = 1
   else
      @ year_corr = b = 0
   end
   if (month <= 2) then
      @ year--
      @ month += 12
   end
   # Cope with the Gregorian calendar reform
   if (year * 10000 + month * 100 + day > 15821015) then
      @ a = year//100
      @ b = 2 - a + a//4
   end
   @ a = floor(365.25 * year - year_corr) + ^
      floor(30.6001 * (month + 1)) + day + 1720994 + b
   # 4713 BC was not a leap year.
   if (a < 59) @ a++
   return a
end

julian $argv