> If a leap second is to be inserted, then in most Unix-like systems the OS kernel just steps the time back by 1 second at the beginning of the leap second, so

> If a leap second is to be inserted, then in most Unix-like systems the OS kernel just steps the time back by 1 second at the beginning of the leap second, so the last second of the UTC day is repeated and thus duplicate timestamps can occur. So certain unix timestamp can represent two different points in time, the last normal second of the day or the leap second that happened after it. This makes (certain) unix timestamps ambiguous.