=a1+120
Remember to format the cell as a date.

And just something that you didn't ask about:

To add 4 months to a date in A1:
=date(year(a1),month(a1)+4,day(a1))

It may have trouble on days near the end of the month.

Needananswer wrote:

Is there a way to reference another cell in a date value formula? Example
cell A1 contains a date. In Cell A5 I need to use the date value formula to
add 120 days to the date in cell A1. The date value formula seems to need an
exact date ("06/30/2007") to calculate the formula. How can I get around
this?

Thanks for you help.

--

Dave Peterson