Hi,
Without thinking too much, how about this:
Code:
Hours VAR BYTE
Minutes VAR BYTE
Seconds VAR BYTE

Hours = 1
Minutes = 1
Seconds = 0
Ticks = 100

ISR:
  Ticks = Ticks - 1
  If Ticks = 0 THEN              ' Ticks rolled over, one second has passed
    Ticks = 100                    ' Reset ticks (10ms ticks)

    Seconds = Seconds - 1     ' Count down a second.

    If Seconds = 255 THEN      ' Seconds rolled over
      Seconds = 59                 ' Reset seconds
      Minutes = Minutes - 1      ' Count down a minute

        If Minutes = 255 THEN  ' Minutes has rolled over
          Minutes = 59              ' Reset minutes
          Hour = Hour - 1         ' Count down an hour
        ENDIF
    ENDIF
  ENDIF
/Henrik.