Coordinated Universal Time with Smoothed Leap Seconds (UTC-SLS)

Document Type Expired Internet-Draft (individual)
Author Markus Kuhn 
Last updated 2006-01-18
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Coordinated Universal Time (UTC) is the international standard timescale used in many Internet protocols. UTC features occasional single-second adjustments, known as "leap seconds". These happen at the end of announced UTC days, in the form of either an extra second 23:59:60 or a missing second 23:59:59. Both events need special consideration in UTC-synchronized systems that represent time as a scalar value. This specification defines UTC-SLS, a minor variation of UTC that lacks leap seconds. Instead, UTC-SLS performs an equivalent "smooth" adjustment, during which the rate of the clock temporarily changes by 0.1% for 1000 seconds. UTC-SLS is a drop-in replacement for UTC. UTC-SLS can be generated from the same information as UTC. It can be used with any specification that refers to UTC but lacks provisions for leap seconds. UTC-SLS provides a robust and interoperable way for networked UTC- synchronized clocks to handle leap seconds. By providing UTC-SLS instead of UTC to applications, operating systems can free most application and protocol designers from any need to even know about UTC leap seconds.


Markus Kuhn (

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