Leap Second
leap-second
A one-second adjustment applied to UTC to keep it aligned with the Earth's irregular rotation.
Leap Smear
leap smear
Overview
Leap smear is a method for handling leap seconds without introducing the normally non-existent timestamp 23:59:60. Instead of inserting a sudden extra second, the adjustment is spread across a window of time by making each second very slightly longer. Google pioneered this approach in 2008, and it has since been adopted by other cloud providers, including Amazon (AWS).
How It Works
In Google's implementation, each second within a 24-hour window (12 hours before and after the leap second) is lengthened by 1/86,400 of a second, approximately 11.6 microseconds. Over 86,400 seconds, these tiny additions sum to exactly one full second. During the smear window the system clock drifts up to 0.5 seconds from true UTC, but at no point does an anomalous 23:59:60 timestamp appear.
Provider Differences and Caveats
The start time and duration of the smear window differ among providers. Google and AWS both use a 24-hour smear, though their start times may not be identical. Other implementations spread the adjustment over a shorter window. When comparing timestamps across different cloud environments around a leap second event, discrepancies of up to one second can occur due to these differing smear schedules. In a 2022 resolution, the General Conference on Weights and Measures (CGPM) decided that the maximum permitted difference between UT1 and UTC will be widened by 2035. The new limit and its implementation date were still undecided as of September 2026, but once the limit is widened, leap second insertions will effectively end and leap smearing will no longer be needed.
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Related Terms
NTP (Network Time Protocol)
ntp
A networking protocol for clock synchronization between computer systems over packet-switched networks.
UTC (Coordinated Universal Time)
utc
The primary time standard by which the world regulates clocks and time, serving as the basis for civil timekeeping globally.
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