Please help

All I want to do is write (record and edit) a macro in
excel that will repeat the calculate (F9) function every 5
seconds for a set period i.e. an hour.

I am really sure this is very easy if you know how. This
is for use in a small countdown program that at the moment
I manually have to hit the F9 key.

Can someone guide me, thanks in advance.

wooders