This is a great little how-to on hiding and unhiding cells in Microsoft Excel: http://spreadsheets.about.com/od/excelformatting/qt/070814unhidecol.htm
It seems Excel still can’t tell me the days in a particular month.
The following formula works, though:
Obviously, A1 has to have a date in it.
Works like a charm:
Update, much, much later: Looks like LibreOffice has a DAYSINMONTH function.