The first <a href="https://en.wikipedia.org/wiki/Leap_second" id="mwAS0" rel="mw:WikiLink" title="Leap second">leap second</a> is added to the <a class="mw-redirect" href="https://en.wikipedia.org/wiki/UTC" id="mwAS4" rel="mw:WikiLink" title="UTC">UTC</a> time system.