Convert a calendar date to true Julian day

INTERFACE:

    subroutine julian_day(yyyy,mm,dd,julian)

DESCRIPTION:

Converts a calendar date to a Julian day. Based on a similar routine in Numerical Recipes.

USES:

    IMPLICIT NONE

INPUT PARAMETERS:

    integer                             :: yyyy,mm,dd

OUTPUT PARAMETERS:

    integer                             :: julian

REVISION HISTORY:

    Original author(s): Karsten Bolding & Hans Burchard