Public Function Months( _
ByVal datDate1 As Date, _
ByVal datDate2 As Date, _
Optional ByVal booLinear As Boolean) _
As Integer
' Returns the difference in full months between datDate1 and datDate2.
'
' Calculates correctly for:
' negative differences
' leap years
' dates of 29. February
' date/time values with embedded time values
' negative date/time values (prior to 1899-12-29)
'
' Optionally returns negative counts rounded down to provide a
' linear sequence of month counts.
' For a given datDate1, if datDate2 is decreased stepwise one month from
' returning a positive count to returning a negative count, one or two
' occurrences of count zero will be returned.
' If booLinear is False, the sequence will be:
' 3, 2, 1, 0, 0, -1, -2
' If booLinear is True, the sequence will be:
' 3, 2, 1, 0, -1, -2, -3
'
' If booLinear is False, reversing datDate1 and datDate2 will return
' results of same absolute Value, only the sign will change.
' This behaviour mimics that of Fix().
' If booLinear is True, reversing datDate1 and datDate2 will return
' results where the negative count is offset by -1.
' This behaviour mimics that of Int().
' DateAdd() is used for check for month end of February as it correctly
' returns Feb. 28. when adding a count of months to dates of Feb. 29.
' when the resulting year is a common year.
'
' 2010-03-30. Cactus Data ApS, CPH.
Dim intDiff As Integer
Dim intSign As Integer
Dim intMonths As Integer
' Find difference in calendar months.
intMonths = DateDiff("m", datDate1, datDate2)
' For positive resp. negative intervals, check if the second date
' falls before, on, or after the crossing date for a 1 month period
' while at the same time correcting for February 29. of leap years.
If DateDiff("d", datDate1, datDate2) > 0 Then
intSign = Sgn(DateDiff("d", DateAdd("m", intMonths, datDate1), datDate2))
intDiff = Abs(intSign < 0)
Else
intSign = Sgn(DateDiff("d", DateAdd("m", -intMonths, datDate2), datDate1))
If intSign <> 0 Then
' Offset negative count of months to continuous sequence if requested.
intDiff = Abs(booLinear)
End If
intDiff = intDiff - Abs(intSign < 0)
End If
' Return count of months as count of full 1 month periods.
Months = intMonths - intDiff
End Function
