Online Stopwatch with Lap Times and CSV Export

The stopwatch is built on two browser primitives: performance.now(), the W3C High Resolution Time API, and requestAnimationFrame, the standard render-synchronised callback. Together they give a clock that stays accurate even when the tab is backgrounded.

When you press Start, the tool records the currentperformance.now() value as the run's start timestamp. On every animation frame, the live elapsed time is computed as elapsed = accumulated + (now − start) and rendered to the screen. Because we store a single start timestamp rather than counting ticks, the elapsed value is always correct regardless of how many frames the browser actually delivered — background tabs, throttled windows, and reduced-motion users all see the same final reading.

When you press Pause, the elapsed time is folded into accumulated and the start timestamp is cleared. Resume starts a fresh interval that adds to the saved accumulator, so pauses are lossless and unlimited.

A Lap snapshots the current elapsed time as that lap's split. Each lap also stores its lap time— the difference between its split and the previous lap's split. The sum of all lap times equals the most recent split, by construction, so the table is self-checking.

The CSV export follows RFC 4180: CRLF line terminators, double-quoted fields where the content contains a comma or quote, and a UTF-8 BOM-free body. The file includes both formatted columns (HH:MM:SS.mmm) and raw millisecond columns so you can re-format the times in your spreadsheet of choice.

The on-screen readout truncates rather than rounds. A true reading of 999 ms is shown as 00:00.99, never 00:01.00 — a stopwatch that lies about completed time is a stopwatch nobody can trust for race timing.

• W3C — High Resolution Time, Level 3 (Performance.now spec)
• MDN — Performance.now() reference
• MDN — window.requestAnimationFrame reference
• IETF RFC 4180 — Common Format and MIME Type for CSV Files

The clock and CSV formatter were last cross-checked against these references on 2026-05-11.