COUNTNUMBEROFDAYS

There are 2 functions.
For instructions on how to add a function to a workbook refer to the page under Inserting Functions

COUNTNUMBEROFDAYS_INAMONTH

Returns the number of days in a month.

Public Function DAYSINAMONTH(ByVal iMonthNumber As Integer, _ 
                           ByVal iYear As Integer) As Integer 
   DAYSINAMONTH = VBA.Day(VBA.DateSerial(iYear, iMonthNumber + 1, 1) - 1) 
End Function 

COUNTNUMBEROFDAYS_INAYEAR

Returns the number of days in a particular year.