Excel lacks a function for calculating the number of days in a particular month, so you'll need to construct your own formula. If cell A1 contains a date, this formula will return the number of days in the month: =day(date(year(A1),month(A1)+1,1)-1).

1-2-3's @dateinfo function is more helpful. The second argument for this function determines the type of information to return about a date (oddly enough, "7" yields the number of days in the month). This formula refers to a date in cell A1: @dateinfo(A1,7).