How to leave Date formula cell blank until date entry??

From limited experience I know that excel calculates dates via serial numbers.

I have formulas to add a number of days to a cell containing an entered date and display the resulting new date (ie. 03/01/2011 (c34)+11 = 03/12/2011)

I am seeking to leave resulting formula cell blank until a date is entered in the input cell. Currently when the input cell is empty the formula cell obviously displays 1/11/1900 using the above example.

What conditional format would achieve leaving the formula cell blank until date data in entered into the source cell?

Hopefully a simpler question for your experience level than mine.