[fanf2]

At most half a second, in the middle of the smear at midnight when the leap second is applied.

The Facebook smear is asymmetrical so it starts off 1 second off just after the leap second, and subsequently corrects itself.

[ The reason Google and Amazon use a linear smear is because NTP clients try to measure the rate difference between their local clock and the reference clocks; if that is different every time the NTP client queries its servers, it will have trouble locking on and accurately matching the smear curve. You can mitigate this somewhat by fixing a higher NTP query frequency, but that’s a heavy-handed fix for an engineering mistake. ]