Formula results giving 29/01/1990?

Hi,

I have a very basic spreadsheet, in it are 4 columns:

Name of a member of staff
The date of the member of staff's first day off
The date of the last day off
The date which they need to return a form (29 days after the last holiday day)

I have created the formula in the last column to work out the return form date, the formula is:

For example: =C2+29

This gives me back the date correctly. However, the spreadsheet also contains empty rows where staff have not yet taken a holiday. When copying the formula down the last column, staff who have not taken days off have a return form date of 29/01/1900.

Is there a way to get Excel to only activate the formula (for want of a better phrase) to only work out the due date when figures are put into the other staff's rows?

I've attached the sheet for a better example of my meaning.

Thanks.