You can try something like =IF(C2="","",C2+28) in D2, that looks to see if there is anything in C2 and if there isn't it clears the cell. However if they put something in there that isn't a date you'll get #VALUE!. To prevent this, if the only thing you want in there is a date, you can click on C2, then click on data menu, then click Data Validation. A pop up should come up, set Allow: to "Date", Data to "greater than" and then Start Date to 1\1\2014 or what ever date you want. Now when someone puts something in there other than a date, they will get a popup that says the value they entered is not valid. they can retry, cancel or select help. If you put both of these together you should have what you're looking for.