Many consumer alarm clocks have a snooze button that is
calibrated for 9 minutes.Does that sound like an odd
decision? Many people would think a 10-minute snooze
interval would make more sense.So were the designers asleep?
Why 9 minutes?
By setting the snooze time to 9 minutes, the alarm clock only needs to
watch the last digit of the time. Whether implemented by physical circuitry
or software, it is easier to manipulate just the terminal digit.