|You can use the =EOMONTH() function to get what you want.|
If you have todays date in cell A1
then in cell B1 enter:
and it will return March 31, 2010 or the end of the month date.
If all you want is the number of days in the month then something like:
will give you the number 31
Just remember to format your cell to reflect the type of info your looking for. A date format or a number format.
Also, the EOMONTH function is part of the Analysis ToolPak, look in your TOOLS - ADD-INS and make sure the Analysis ToolPak is checked.