What a leap second actually is
Earth's rotation is slowing. To stop official noon drifting away from the sun's highest point, the people who maintain Coordinated Universal Time (UTC) occasionally insert an extra tick. The International Earth Rotation and Reference Systems Service (IERS) calls the shots. The last insertion landed at midnight UTC on December 31, 2016. None has been announced since.
Why your browser clock ignores it
Your browser is not a time source. It reads whatever your operating system reports. When an extra second arrives, the OS absorbs it before the browser ever sees it. Two methods dominate.
Smearing: the OS stretches the day's final 1,000 milliseconds across 1,001 milliseconds of real time. Each tick lengthens by a fraction. The browser notices nothing.
Ignoring: the OS treats the leap as a duplicate. The clock shows "23:59:59" for two real seconds, then flips to "00:00:00". The display pauses. It never jumps backward.
No consumer OS or browser will show "23:59:60", the official leap notation. The adjustment is deliberately hidden.
What happens to online timers and stopwatches
A stopwatch built correctly does not touch Date.now(). It uses performance.now(), a monotonic clock that only ever increases. System clock adjustments, DST transitions, leap seconds: none of them touch it.
A live clock uses Date.now(), which is wall-clock time. Wall-clock time can jump from NTP corrections or manual changes. A single inserted tick is too small and too rare to register. The display ticks forward. The extra moment passes. Nothing breaks.
The practical answer: your countdown, your stopwatch, your live clock will not glitch, skip, or stutter on a leap second.
The next leap second is not scheduled
As of October 2026, no leap second has been announced. The IERS gives months of notice. If one comes, it will land at the end of June or December.
When that day arrives, your browser clock will behave exactly as it does now. No "23:59:60". The OS will smear or repeat the moment, and the clock will move on.
Do leap seconds matter for my browser clock?
No. The gaps between NTP syncs cause far more drift. Consumer hardware can wander 1 to 15 seconds per month. That is the real problem, not a single tick inserted once every few years.
For streaming, exam timing, fitness intervals, meeting countdowns: the leap second is invisible. The one exception is precision time-stamping for legal or scientific records. In that case, a dedicated time source is already mandatory, not a browser.