If you’re the kind of person who likes to close a bar on a Tuesday night, then good news for you! There are discrepancies between the amount of time it takes us to round the sun and the clocks we use to measure it. In order to correct those discrepancies, we add a second every once-in-a-while. That once-in-a-while is now! We add a second at midnight.

If this was a normal night, the sequence to midnight would look like this:

  2015-06-30 23.59.57
  2015-06-30 23.59.58
  2015-06-30 23.59.59
  2015-07-01 00.00.00
  2015-07-01 00.00.01

But this ain’t no normal night, mister. Tonight we add an additional second. That sequence looks like this:

  2015-06-30 23.59.57
  2015-06-30 23.59.58
  2015-06-30 23.59.59
  2015-06-30 23.59.60 <-- leap second
  2015-07-01 00.00.00
  2015-07-01 00.00.01

So if closing time arrives at Midnight, you can say to the bartender, “Not so fast, Jeeves! I’ve got another second!”

But let’s say — and why the hell not? — that instead of drinking beer, you like testing servers into the wee hours of the morning. What is siege going to do at 23:59:60? To be honest, I have no idea. Chances are your server won’t leap until its next NTP update. All transactions that occur during the adjustment will probably be skewed a second too long. (It might be a good idea to run the update manually)

NOTE: So how do you sync your Linux laptop with one of the government’s atomic clocks? Just use the time server at the National Institute of Standards. You can do that with the ntpdate command like this:

Bully # ntpdate time.nist.gov 
30 Jun 17:34:36 ntpdate[3977]: adjust time server 128.138.141.172 offset -0.026286 sec