Time scales and civil time
Leap second
Also: schrikkelseconde
The Earth's rotation slows down slightly and irregularly, causing astronomical time (UT1) to drift slowly away from atomic time. To keep UTC within 0.9 seconds of UT1, the IERS inserts a leap second from time to time (23:59:60 UTC), usually on 30 June or 31 December.
Since January 2017 the count stands at 37 leap seconds. Systems that do not handle the extra second correctly may produce errors at that moment.