Take a look at this Elapsed Timer Demo.

http://www.picbasic.co.uk/forum/showthread.php?t=190

Then, all you need to do is this...
Code:
Loop:
    if (DaysChanged= 1) and (Days = 40) then
        DaysChanged= 0
        Gosub ResetTime
        ' ... Do something here ...
    endif
Goto Loop