Google Cloud Platform Blog
Making every (leap) second count with our new public NTP servers
Wednesday, November 30, 2016
Posted by Michael Shields, Technical Lead, Time Team
As if 2016 wasn’t long enough, this year, a
leap second
will cause the last day of December to be one second longer than normal. But don’t worry, we’ve built support for the leap second into the time servers that regulate all Google services.
Even better, our
Network Time Protocol
(NTP) servers are now publicly available to anyone who needs to keep local clocks in sync with VM instances running on
Google Compute Engine
, to match the time used by Google APIs, or for those who just need a reliable time service. As you would expect, our
public NTP service
is backed by Google’s
load balancers
and atomic clocks in data centers around the world.
Here’s how we plan to handle the leap second and keep things running smoothly here at Google. It’s based on what we learned during the leap seconds in 2008, 2012 and 2015.
Leap seconds compensate for small and unpredictable changes in the Earth's rotation, as determined by the
International Earth Rotation and Reference Systems Service (IERS)
. The IERS typically announces them six months in advance but the need for leap seconds is very irregular. This year, the leap second will happen at 23:59:60 UTC on December 31, or 3:59:60 pm PST.
No commonly used operating system is able to handle a minute with 61 seconds, and trying to special-case the leap second has caused
many problems
in the past. Instead of adding a single extra second to the end of the day, we'll run the clocks 0.0014% slower across the ten hours before and ten hours after the leap second, and “smear” the extra second across these twenty hours. For timekeeping purposes, December 31 will seem like any other day.
All Google services, including all APIs, will be synchronized on
smeared time
, as described above. You’ll also get smeared time for virtual machines on Compute Engine if you follow
our recommended settings
. You can use non-Google NTP servers if you don’t want your instances to use the leap smear, but don’t mix smearing and non-smearing time servers.
If you need any assistance, please visit our
Getting Help
page.
Happy New Year, and let the good times roll.
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