you can't add 60 days to a cell with date data already in the cell.
IE: Cell A1 contains the date: 06/15/2011
You can add 60 days to the date and have the result in another cell.
IE: Cell A1 contains the date: 06/15/2011 and Cell B1 =A1+60
To increment the date in cell A1 you either have to manually update it, or get a piece of VBA code to do it. Sorry, my VBA code skills are nill.
If you want the second scenario then try this
In cell B1 enter: (DerbyDad03 created this one)
This will insure that cell A1 contains a Date, not just any data.