Julian day functions

This code has been declared by its author to be Public Domain code.

Julian day functions by Craig Watson

2002

This demonstrates a simple DateDiff() type function using the Julian Day. You can also use it for things like determining the day of the week (try Julian Date MOD 7)

Print BlitzDateToJulianDay("12 Dec 2002") - BlitzDateToJulianDay(CurrentDate$()) + " Days until my birthday"
WaitKey()

Function BlitzDateToJulianDay(datestr$)
    ;break up a blitz style date into d/m/y then get the JD
    dy = Left(datestr$, Instr(datestr$, " ") - 1)
    yr = Mid(datestr, Instr(datestr, " ",Instr(datestr$, " ")+1)+1, 4)
    mon$ = Mid(datestr, Instr(datestr, " ")+1, 3)

    Select Upper(mon)
        Case "JAN"
            mn = 1
        Case "FEB"
            mn = 2
        Case "MAR"
            mn = 3
        Case "APR"
            mn = 4
        Case "MAY"
            mn = 5
        Case "JUN"
            mn = 6
        Case "JUL"
            mn = 7
        Case "AUG"
            mn = 8
        Case "SEP"
            mn = 9
        Case "OCT"
            mn = 10
        Case "NOV"
            mn = 11
        Case "DEC"
            mn = 12
    End Select

 	Return JulianDay%(dy, mn, yr)	
End Function

Function JulianDay%(d%, m%, y%)

	If m% < 3 Then
      m% = m% + 12
      y% = y% - 1
	End If
	
	Return d% + Floor(153 * m% - 457) / 5 + 365 * y% + Floor(y% / 4) - Floor(y% / 100) + Floor(y% / 400) + 1721118.5
End Function

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