秒表和倒计时器

Two timing tools in one

This tool combines a stopwatch that counts up from zero and records laps, and a countdown timer that counts down to a time you set and alerts you when it reaches zero. Both run entirely inside your browser tab with no installation, no account, and no server. The difference between them is direction: a stopwatch measures how long something takes, while a countdown enforces a fixed duration. Knowing which one fits your task — and how each keeps time under the hood — helps you trust the numbers it shows.

How the stopwatch measures time accurately

The stopwatch is driven by performance.now(), a high-resolution clock that returns the time in fractions of a millisecond. Rather than incrementing a counter on a fixed interval (which would drift), the tool records the moment you press Start and, on every screen refresh, subtracts that start point from the current time. The display updates in sync with your monitor's refresh rate via requestAnimationFrame, so it looks smooth, but the underlying figure is always the true elapsed time — never an accumulation of rounding errors. The readout is formatted as minutes, seconds, and three-digit milliseconds, for example 02:14.837.

Laps: split time versus total time

Press Lap while the stopwatch runs and it captures two numbers. The total is the elapsed time since you started. The split is the time since the previous lap — the duration of just that segment. This distinction is the whole point of lap timing.

Here the runner's second segment (1:28.5) was the fastest, even though the total kept rising. Reading splits, not totals, tells you where you sped up or slowed down. Use Copy Laps to export the full list as tab-separated text you can paste into a spreadsheet.

How the countdown works and what the alert is

In countdown mode you enter hours, minutes, and seconds, then press Start. The tool sums them into a total number of seconds and ticks down once per second using setInterval. When it reaches zero it stops, shows a "Time's up!" banner, and plays a short built-in beep encoded directly in the page — there is no external sound file to download, so the alert works offline. Hours are capped at 99 and the minute and second fields at 59, matching normal clock notation.

Practical use cases

• Workout intervals. Set a 45-second countdown for an exercise, then a 15-second one for rest.
• The Pomodoro technique. A 25-minute countdown for focused work, a 5-minute one for breaks.
• Cooking and brewing. A 4-minute countdown for steeping tea or a soft-boiled egg.
• Timed practice. Use the stopwatch with laps to time each rep, problem, or recitation and compare splits.
• Public speaking. Run the stopwatch to keep a talk within its slot.

The one accuracy caveat: background tabs

The most important limitation to understand is browser tab throttling. To save battery, browsers slow down timers in tabs that are not visible. If you switch to another tab or minimize the window, both the stopwatch animation and the countdown interval may run less frequently than once per second. The countdown can therefore finish a little late if it was hidden, and the alert may only sound when you return to the tab. For anything that must be exact while you look away, keep this tab in the foreground or use a dedicated device timer.

Common mistakes

• Expecting laps to be saved. Laps live only in memory. Closing or refreshing the page erases them, so copy them out before you leave if you need a record.
• Confusing Pause with Reset. Pausing keeps your elapsed time and laps so you can resume; Reset clears everything back to zero.
• Starting a countdown with all zeros. If you do not enter a duration, there is nothing to count down and Start does nothing — set at least one of hours, minutes, or seconds first.

Privacy

Everything runs locally in your browser. No timing data, lap, or setting is ever sent to a server — the tool functions fully offline.