Right, but this uses a 30-second period, which could be affected by smaller variations. In this case the use of the TOD is unavoidable because if I used the Timer A interrupt (50Hz or 60Hz depending on video standard), it would be slowed when interrupts were disabled for doing BCD calculations - the C64's IRQ routine at $ea31 can't handle the situation if an IRQ occurs while decimal mode is on. This is a known Kernal bug. I suppose I could hack around that but this was simpler.