Every six months or so, this neat group called the International Earth Rotation Service issues a directive on whether there will be a leap second inserted at the end of that six month period. You usually find out at the beginning of January or the beginning of July, and thus would have a leap second event at the end of June or December, respectively. Ten years ago, in January 2015, they announced a leap second would be added at the end of June 2015. The last one had been three years prior, and...