It seems Excel still can’t tell me the days in a particular month.
The following formula works, though:
=DAY(DATE(YEAR(A1),MONTH(A1)+1,1)-1)
Obviously, A1 has to have a date in it.
Works like a charm:
OpenOffice.org doesn’t seem to have any built-in functions for cube roots, but there’s an easy workaround. To find the nth root, use powrs to the nth fraction. Probably easier to illustrate than to explain with my rusty math skills. 
I mean this:
You need to use the brackets because otherwise the power would take precedence over the division.
