Will my city page ever show 23:59:60?

No. Civil displays here are ordinary minutes derived from IANA offsets. Leap seconds are a UTC implementation detail.

Do leap seconds cause DST bugs?

No. DST bugs come from wrong zone ids and mismatched change weekends. Leap-second bugs come from APIs that assume every minute has 60 seconds.

Is Unix time the same as UTC?

Unix time counts seconds since 1970-01-01 UTC with leap seconds typically smeared or skipped in POSIX. It is a computing convention, not a civil zone.

Leap seconds and civil time

City clocks on NowZones never show a 23:59:60. Civil time for people is IANA plus a UTC offset. Leap seconds live in the time-scale that keeps atomic clocks close to the spinning Earth.

Last updated 2026-08-20 · Written by the NowZones publisher in Denmark. Offsets and DST examples use the IANA Time Zone Database via the same libraries as the city clocks. This is editorial explanation, not legal, medical, or aviation advice.

What a leap second is

UTC is an atomic time scale. Earth’s rotation is slightly irregular, so UTC is occasionally stepped by one second to stay near UT1 (astronomical time). That step is a leap second, usually inserted at 23:59:60 UTC on 30 June or 31 December.

IANA zone offsets are defined against UTC without exposing that extra second on a world-clock page. Your phone jumping from 00:59:59 to 01:00:00 in London is a civil minute; the leap, if any, already happened in the UTC layer underneath.

Unix time and many databases cannot represent 23:59:60. They smear the second or repeat a timestamp. That is why distributed systems have outages on leap-second nights, not why Tokyo’s office clock is “wrong.”

Leap seconds are being retired

In 2022 the General Conference on Weights and Measures agreed to stop inserting leap seconds by 2035 (the exact mechanism of future UTC is still being specified). The point is to stop breaking computers.

Civil timezone rules — when the US springs forward, when India stays on +05:30 — are unrelated. Those stay in tzdb. Leap seconds were never how NowZones computed “09:00 in Delhi.”

If you operate GNSS, telecom, or a trading matching engine, you already have a leap-second runbook. If you schedule humans, you can ignore this article except as context for why UTC is not “just GMT.”

UTC, GMT, and this site

For meeting math, UTC and GMT agree to within a second. We still say UTC on fact blocks because that is what IANA offsets are defined against. The UTC vs GMT guide covers the naming.

Zulu time in aviation is UTC. It does not observe DST and it does not show leap seconds on a dispatch strip either; crews care about the zone conversion, not the 61st second.

Try the tools

Frequently asked questions

Will my city page ever show 23:59:60?: No. Civil displays here are ordinary minutes derived from IANA offsets. Leap seconds are a UTC implementation detail.
Do leap seconds cause DST bugs?: No. DST bugs come from wrong zone ids and mismatched change weekends. Leap-second bugs come from APIs that assume every minute has 60 seconds.
Is Unix time the same as UTC?: Unix time counts seconds since 1970-01-01 UTC with leap seconds typically smeared or skipped in POSIX. It is a computing convention, not a civil zone.