What do you have so far? You could do this with conditional formatting or
something like this to define the date 21 days (calendar) out

=date(year(A1),month(A1),day(A1))+21

Or a combination of the two

"Wanderer" wrote:

I am trying to set up a spreadsheet that will flag me 21 days from the date I
send out documents. So that I can follow up on those documents.