I don't know if it's relevant, but it occurred to me that RTC crystals used in embedded systems are tuned by manufacturers to operate at the very specific frequency of 32.768kHz. If I recall the original reasoning was that the MSB of a 16-bit counter value toggles every 1 sec for that frequency, and now I'm guessing it's just kind of legacy that it continues to be done that way instead of going to some faster multiple of that value (which would mean physically smaller crystals, less material, board space, etc.). Kind of a moot point since modern technology tends to move away from traditional RTC crystals anyway, but I digress.

I mention the above because the period for the 32.768kHz frequency is about 30.5 microseconds, so the choice of 20 hours for the smear could have been as arbitrary as the number of hours necessary to reach less than half that value? Technically 19 hours would have been enough, but who likes odd numbers? Besides, us silly engineers have this always add in a margin of error habit even in the digital/discrete realms where we know darn well it doesn't matter. Granted, I'm completely speculating on the reason for the 20 hour smear, I could be totally wrong, I have no inside knowledge or anything like that.