閏秒

Google Public NTP は leap smear する
https://developers.google.com/time/
Google Public NTP serves leap-smeared time.

Cloudflare's Time Service はしない
https://developers.cloudflare.com/time-services/ntp/usage/
We do not implement leap smearing: NTP includes a Leap Indicator field spec and the kernel will apply the leap second correction at the appropriate time. This is the behavior servers in pool.ntp.org share. Using servers that smear time along with servers that do not may lead to unpredictable and anomalous results.