Binaryworld - Tricks with DateSerial ... [ VB -> Date-Time ]

The DateSerial function has an interesting feature: it doesn't raise errors when you pass it an invalid month or day number. Instead, it evaluates the date as if the arguments were valid. For example, DateSerial(2000, 1, 32) returns the Date value of February 1, 2000. This behavior can (and should) be considered as a bug, but it's a fact that it has never changed since VB1 and it is highly unlikely that Microsoft will change it in future versions of VB.

You can take advantage of this feature by writing date functions that would be rather difficult to write otherwise. For example:

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' the last day of a givan month
Function EndOfMonth(Year As Integer, Month As Integer) As Date
  EndOfMonth = DateSerial(Year, Month + 1, 0)
End Function

' number of days in a month
Function DaysInMonth(Year As Integer, Month As Integer) As Integer
  DaysInMonth = Day(DateSerial(Year, Month + 1, 0))
End Function

' True if leap year
Function IsLeapYear(Year As Integer) As Boolean
  IsLeapYear = (Day(DateSerial(Year, 3, 0)) = 29)
End Function