Working with empty cells

How do I tell excel that a certain calculation should be done when the data cells contributing to the formula are filled. Example, I have a cell which contains a date. Another cell contains a number. When I add the two up I should get another date. By default when no information is filled in the date cell a default value of Jan-00 is filled in that cell because the formula sees the date cell containing no value and thus equal to zero. Is it possible to adapt the formula to say that a certain function is to be carried out only if a date cell has a date in it?