Assuming you have entered your date in A1 then this formula should do it;
=DAY(DATE(YEAR(A1),MONTH(A1)+1,0))-DAY(A1)+1
Format the cell holding the formula as general.

Hope this helps
Rowan

if function for remaining days in month wrote:
Hi I need to know if anyone can give me an IF Function that will calculate
the number of days remaining in a given month. For example, if the date is
July 17th, the calculation would be 31-17+1=15 days. My problem is setting
the citeria to recognize whether the month is a 31 day month/30 day month or
28/29 day month (I would prefer not having to enter in that information for
each date). Please help! Thank you!