X axis - time of day

Y axis - NTP server's delta from actual time

What you're seeing with the yellow lines is "leap smearing" by Google's public NTP time servers. They slowly diverge from the real time for the 10 hours before and after the leap second, so that they can have 60 instead of 61 seconds at midnight. See: https://cloudplatform.googleblog.com/2016/11/making-every-le....