@Echo Off
:: GREGOR.BTM Convert Julian Day Number to Gregorian Date
:: Author: Michael Bednarek (mb@doh.health.nsw.gov.au)
:: Input:  nnnnnnn
:: Output: Echo DOW and GREGOR and return variable GREGOR
:: Method: From a German HP Calculator Manual
::         which says it only works for dates between 1-Mar-1900 and 28-Feb-2100
::         Seems to work longer, though.

:: Parameter is not checked!

:: bias to 1-Jan-4713 BC
Set bias=1720983

:: Compute preliminary year and month, and day
Set y2=%@Int[%@Eval[(%1 - %bias - 122.1) / 365.25]]
Set m2=%@Int[%@Eval[(%1 - %bias - %@Int[%@Eval[365.25 * %y2]])/30.6001]]
Set dd=%@Eval[%1 - %bias - %@Int[%@Eval[365.25 * %y2]] - %@Int[%@Eval[30.6001 * %m2]]]

:: Correct month and year
Iff %m2 gt 13 Then
  Set mm=%@Eval[%m2 - 13]
  Set yy=%@Eval[%y2 + 1]
Else
  Set mm=%@Eval[%m2 - 1]
  Set yy=%y2
EndIff

:: Done
Set GREGOR=%dd %mm %yy
Set DOW=%@Word[%@Eval[%1 %% 7],Sun,Mon,Tue,Wed,Thu,Fri,Sat]

:: Clean up
Unset y2 m2 dd mm yy bias

:: Echo only if called from the command line
If %_BATCH eq 1 Echo %GREGOR %DOW