From what i understand, you want to execute a Subroutine at every 15minutes right?

The only thing that spring to mind without adding extra hardware is a Timer interrupt.. or measure the mainloop time, execute it untill it reach the 15Minutes, then execute the Subroutine. Once you know the time of the main program, you just calculate the amount of time you'll have to repeat it.

If you don't know the time... you can even use a extra i/o and measure it with a scope or whatever else.

Snip
High ExtraI_O
'mainRoutine
Low ExtraI_O

Assuming you must loop 'x' time

Code:
Start:
    for Repeat=0 to x
        ' MainLoop
        Next

    'Do your Subroutine here
    GOTO Start