Вот метод, который вы могли бы использовать:
Public Shared Function MonthDifference(ByVal first As DateTime, ByVal second As DateTime) As Integer
Return Math.Abs((first.Month - second.Month) + 12 * (first.Year - second.Year))
End Function
вот так:
Dim Date1 As New DateTime(2010,5,6)
Dim Date2 As New DateTime(2009,10,12)
Dim NumOfMonths = MonthDifference(Date1, Date2)