A Leap Smear is a technique of spreading or "smearing" time over a period of hours to account for leap seconds. Leap seconds are periodical adjustments that result in the addition of a second to the Coordinated Universal Time (UTC) to keep UTC time synchronized with the Earth's ever-slowing rotation.
These leap seconds can create issues for IT systems and software code that require time synchronization with other systems for accurate results. When an application or service interacts with another system that hasn't accounted for a leap second (or vice versa), errors with tracking and reporting events, keeping replications up to date and in sync, determining the order of data operations, and more can result.
To prevent these types of issues, companies like Google, Amazon, Microsoft and Akamai have utilized leap smears in their NTP (Network Time Protocol) services to help prevent the potential for critical IT issues associated with leap seconds.
Google's 20-Hour Linear Leap Smear
For the most recent leap second on December 31, 2016 (as well as in 2012 and 2015), Google used a 20-hour linear smear, in which Google slows down its NTP server clocks 0.0014 percent for 10 hours both before and after the leap second to account for it without disrupting applications and systems that depend on time synchronization.
By configuring network settings to utilize time.google.com as the default NTP server, enterprises can ensure their systems and apps will be able to handle leap seconds. Alternatively, enterprises can access a variety of different NTP services that use other forms of leap smearing to help prevent the potential leap second issues.
Other Leap Smearing Techniques and a Proposal for a Standardized Leap Smear
Unfortunately, many of these NTP services handle leap smearing differently, ranging from Google's 20-hour leap smear to Amazon and Microsoft's 24-hour leap smears, or from Bloomberg's after-the-leap smear over 2,000 seconds to UTC-SLS's before-the-leap linear smear over a 1,000-second span.
Because of these varying leap smear techniques, companies that mix different leap-smearing NTP services could create potential issues for their applications and systems. To help prevent these issues, Google is seeking to move from the 20-hour leap smear it used in 2012, 2015 and 2016, and instead utilize a standardized 24-hour leap smear practice going forward.
This proposed standardized 24-hour linear smear would run from noon to noon UTC, and will be used for the next leap second after the 2016 leap second, which is expected to occur sometime in 2018.
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