dates and lookups

Hi.

Here's the problem.
I'm trying to write a flying logbook worksheet. What i need to do is have a running 28 day total of my flying hours. I simply cant figure out any particular formula that will do the trick.

A bit of info: My worksheet has multiple entries per column for each days flights. ie, column A might contain three entries for three seperate flights.
I tried converting the dates to serial numbers and then subtracting 28 from it etc, but I cant work it out.

Any help?
Thanx zing