[数值问题] 如何得到日期在当年的第几天，即日序

Program fcode_demo
  Implicit none
  integer :: y , m , d , t
  Open(12,File="in.txt")
  Open(13,File="out.txt")
  Do
    read( 12 , * , ioStat = t ) y , m , d
    if ( t/= 0 ) exit
    write( 13 , * ) y , m , d , DaysInYear( y , m , d )
  End Do
  Close(12)
  Close(13)

contains

  Integer Function DaysInYear( year , mon , day )
    Integer :: year , mon , day
    Integer :: DaysInMonth(12) = [31,28,31,30,31,30,31,31,30,31,30,31]
    if ( ( (MOD(year,4)==0).and.(MOD(year,100)/=0) ) .or. (mod(year,400)==0) ) then
      DaysInMonth(2) = 29
    else
      DaysInMonth(2) = 28
    end if
    DaysInYear = sum( DaysInMonth(:mon-1) ) + day
  End Function DaysInYear

End Program fcode_demo