You can use EOMONTH which is included in the Analysis ToolPak. The ToolpPak is not installed by default.
Say you have the date 1-JAN-04 in cell A3. Then in B3 enter the formula
and format cell B3 as a date. You can then drag the formula down.
To load the Analysis ToolPak, Tools/Add-ins and tick Analysis ToolPak.