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Tip of the Day
Language: VB7
Expertise: Intermediate
Jun 30, 2003

GetRoshHashanah - Retrieving the date that Rosh Hashanah begins in the requested year

' Returns the date that Rosh Hashanah begins for the requested year.
' It is important to note that Rosh Hashanah is based on the Lunar cycle so the 
' Holiday actually begins at sunset of the day before the date returned by this 
' function.

' Note: Many dates in the Jewish calendar are dependent on the date of Rosh 
' Hashanah just as Good Friday, Lent, and other dates are dependent on Easter. 

Function GetRoshHashanah(ByVal iYear As Integer) As Date
    Dim dDate As Single, iDate As Integer, iDayOfWeek As Integer
    Dim JY, JtoG, iMonth As Integer

    ' Conversion factors
    Const F1 As Single = 765433 / 492480
    Const F2 As Single = 23269 / 25920
    Const F3 As Single = 765433 / 492480

    JY = (12 * ((iYear Mod 19) + 1)) Mod 19

    ' Correct from Julian date to Gregorian date
    JtoG = iYear \ 100 - iYear \ 400 - 2

    dDate = JtoG + F1 * JY + (iYear Mod 4) / 4 - (313 * CLng(iYear) + 89081) / _
        98496

    ' Get the integer portion of the date
    iDate = CInt(Math.Floor(dDate))
    ' Get the fractional portion of the date
    dDate = dDate - iDate

    If iDate > 30 Then
        iDate = iDate - 30
        iMonth = 10
    ElseIf iDate < 1 Then
        iDate = iDate + 31
        iMonth = 8
    Else
        iMonth = 9
    End If

    iDayOfWeek = (New Date(iYear, iMonth, iDate).DayOfWeek) + 1

    ' Postponement Rules
    ' If day of week is Sunday, Wednesday, or Friday
    If iDayOfWeek = 1 OrElse iDayOfWeek = 4 OrElse iDayOfWeek = 6 Then
        iDate = iDate + 1
        ' If day is Monday
    ElseIf iDayOfWeek = 2 AndAlso dDate > F1 AndAlso JY > 11 Then
        iDate = iDate + 1
        ' If day is Tuesday we may need to add two days
    ElseIf iDayOfWeek = 3 AndAlso dDate >= F2 AndAlso JY > 6 Then
        iDate = iDate + 2
    End If

    If iDate > 30 Then
        iDate = iDate - 30
        iMonth = 10
    ElseIf iDate < 1 Then
        iDate = iDate + 31
        iMonth = 8
    Else
        iMonth = 9
    End If

    Return New Date(iYear, iMonth, iDate)
End Function
Jim Edgar
 
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