one way:

A1 <last ff date
A2 # weeks before last ff date to start

A3: =A1-7*A2

format A3 as a date.

In article ,
Justlearning wrote:

I want to create a seed starting chart in excel, but don't know if I can
create a formula which would allow me to use dates. I want to have a column
where I enter the number of weeks from the first frost free date, and then
have Excel calculate what the seed starting date would be.