Determine number of days left in month

FMP 5
Win xp
FMP Server 5

Given a date field (Close of Escrow Date) how would I determine the
exact number of days left in any given month?

For instance: a client wants a to purchase a home and sign on December
26th.  This client would be responsible to prepay the interest for the
remainder of the month i.e. 5 days.

Thanks for any assistance.

-paul