How many seconds have there been since January 1, 1972?
While that number is extremely large, it is pretty simply calculated. Multiply the number of years (and don’t forget leap years!) by the number of days in a year by the number of hours in a day by the number of minutes in an hour by the number of seconds in a minute and voilà: 1,371,513,600 seconds as of midnight on June 29, 2015.
Except that that’s not right. We’re leaving out leap seconds. Leap seconds, you ask? Yes, leap seconds. There have been 26 of them, and number 27 is just around the corner: It will be added to global clocks at 1am London time on July 1.