Leap Second Calculator

A leap second is an occasional one-second adjustment made to Coordinated Universal Time (UTC) to keep it synchronized with Universal Time 1 (UT1) — the time scale based on Earth's actual rotation. Unlike the predictable leap days added to calendar years, leap seconds are added irregularly because Earth's rotation is not perfectly uniform: it speeds up and slows down due to tidal friction, atmospheric pressure changes, and interactions with Earth's molten core.

Since 1972, when the UTC leap second system was introduced, 27 positive leap seconds have been added (as of 2024), all at 23:59:60 UTC on June 30 or December 31 of their respective years. The leap second makes the UTC clock read 23:59:60 before advancing to 00:00:00 of the new day, giving Earth's rotation a chance to catch up with atomic time.

The international decision was made in 2022 to eliminate leap seconds by 2035. Until then, the Leap Second Calculator shows the full history of all 27 leap seconds, the current UTC-UT1 offset (the Delta T value), and whether the selected date falls on or near a historical leap second insertion.

The calculator finds the cumulative leap seconds for any date by checking the date against a hardcoded table of all historical leap second insertions.

All 27 leap seconds have been positive — adding 23:59:60 before the next minute. No negative leap second (removing a second) has ever been used, though the IERS (International Earth Rotation and Reference Systems Service) has the authority to insert them. The frequency of leap seconds has decreased in recent decades as Earth's rotation has been gradually accelerating:

• 1972–1979: 9 leap seconds — very frequent early on as UTC calibrated to UT1
• 1980–1999: 13 leap seconds — roughly one every 18 months
• 2000–2016: 5 leap seconds — slowing frequency
• 2017–2024: 0 leap seconds — none added in 7+ years

The last leap second was inserted on December 31, 2016. As of 2024, UTC is 37 seconds ahead of UT1 (International Atomic Time is 37 seconds ahead of UTC, which means TAI−UTC = 37s).

1. Select a Date: Enter any date to see the cumulative leap seconds in effect on that date.
2. Check History: The full table of all historical leap second insertions is displayed for reference.
3. Leap Second Day: If the selected date is a historical leap second insertion date, the calculator notes that a leap second was added on that day.

Leap seconds are a significant operational concern for timekeeping infrastructure. When a leap second is inserted, networks, servers, and databases that rely on UTC must handle the discontinuity. A 23:59:60 timestamp does not exist in most computer systems — they may freeze the clock for one second, smear the second across a longer period (Google's "leap smear"), or experience brief anomalies.

Financial exchanges must handle leap seconds carefully because transaction timestamps are legally significant. A trade timestamped 23:59:60 on a leap second day could cause database integrity issues or sorting errors in systems that don't recognize the 60-second value.

GPS receivers, telecommunications networks (NTP/PTP), and astronomical observatories all maintain leap second tables. The decision to discontinue leap seconds after 2035 was made partly because managing them across global infrastructure has become increasingly complex and error-prone.