Leap seconds are needed to reconcile the difference between hyper-accurate atomic time (TAI) and the slightly irregular solar time (UT1) based on the Earth's rotation. When the difference between the two approaches 0.9 seconds, a leap second is added to UTC, usually on June 30 or December 31. The sequence of time becomes 23:59:59 -> 23:59:60 -> 00:00:00. This practice is being phased out and will be ceased by 2035.
What Are Leap Seconds?
A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) to keep it in close alignment with mean solar time (UT1).
Quick Answer
A leap second is an extra second that is sometimes added to our clocks. Because the Earth's rotation isn't perfectly regular, our super-precise atomic clocks can get slightly out of sync with the length of a day. So, about once every year or two, scientists add a leap second to UTC to let the Earth 'catch up,' keeping our clocks accurate.
Related Questions
What is UTC (Coordinated Universal Time)?
UTC is the primary time standard by which the world regulates clocks. It's the successor to GMT and is based on atomic time for precision.
What is Daylight Saving Time (DST)?
DST is the practice of advancing clocks by one hour during warmer months to extend daylight into the evening.
What does GMT stand for?
GMT stands for Greenwich Mean Time, the historic time standard based on the Prime Meridian in Greenwich, London.
How many time zones are in the USA?
The USA and its territories use 9 standard time zones. The 4 contiguous US states time zones are Pacific, Mountain, Central, and Eastern.