Tomorrow, June 30th, a leap second will be added just before midnight UTC (5pm Pacific/8pm Eastern)
Most standard date libraries handle this edge case, but if you are doing any of your own hand rolled date/time processing, you might want to make sure you can handle this edge case (the existence of 23:59:60).
I've been reminding folks here for a month. Our NTP servers are locked and loaded. I'm tempted to hang a webcam in front of one of them and screencap 23:59:60 for the people here who are still confuzzled over the whole concept.