setMilliseconds() writes the sub-second fraction (0–999) within the current local second on a Date object. This guide covers syntax, zeroing fractional noise, overflow, and comparisons with epoch timestamps.
setMilliseconds(n)
Sub-second
Clock fraction
Rolls seconds
In place
Epoch ms
Timestamps from APIs often include fractional seconds. After reading them with getMilliseconds(), setMilliseconds() lets you write that sub-second part in local time.
This is not for measuring elapsed time—for duration, subtract getTime() values. setMilliseconds() adjusts the millisecond hand of the local clock only.
setMilliseconds() MethodDate.prototype.setMilliseconds(milliseconds) accepts an integer. Values 0–999 set the fraction inside the current second. Values outside that range normalize—setMilliseconds(1500) adds one second and leaves 500 ms.
The method mutates the Date and returns epoch milliseconds. For UTC sub-second writes, use setUTCMilliseconds(). To set hour through ms at once, use setHours(h, m, s, ms).
To strip sub-second noise: date.setMilliseconds(0). For exact timestamps: date.setHours(12, 30, 45, 250).
dateObj.setMilliseconds(milliseconds)milliseconds — Integer for the ms within the current local second (overflow normalizes).NaN if the Date was invalid before the call.| Goal | Code |
|---|---|
| Set ms to 500 | date.setMilliseconds(500) |
| Zero sub-seconds | date.setMilliseconds(0) |
| Full time with ms | date.setHours(12, 30, 45, 250) |
| Read ms after set | date.getMilliseconds() |
| Elapsed duration | end.getTime() - start.getTime() |
| Clone before mutate | const copy = new Date(date) |
These APIs involve milliseconds but serve different jobs.
0–999Local sub-second
readGetter pair
epoch msFull instant
now msCurrent timestamp
Open DevTools Console (F12) or use Try-it links. Fixed times use the Date constructor for predictable local values.
Change the millisecond fraction inside the current second.
Update only the sub-second part; seconds and above stay the same.
const date = new Date(2026, 2, 1, 12, 30, 45, 120);
date.setMilliseconds(500);
console.log(date.getSeconds()); // 45
console.log(date.getMilliseconds()); // 500Milliseconds are the fraction inside the current second—not total ms since 1970.
Zero noise, combined setters, return values, and overflow.
Round a timestamp down to whole seconds in local time.
const date = new Date(2026, 2, 1, 12, 30, 45, 789);
date.setMilliseconds(0);
console.log(date.getSeconds()); // 45
console.log(date.getMilliseconds()); // 0Useful before comparing second-granularity times or displaying HH:MM:SS without fractions.
setHours(h, m, s, ms)When you need the full local time including ms, the four-argument overload is often clearer.
const date = new Date(2026, 2, 1, 9, 0, 0, 0);
date.setHours(12, 30, 45, 250);
console.log(date.getHours()); // 12
console.log(date.getMilliseconds()); // 250setHours() accepts ms as its fourth argument when you want one coordinated update.
Preserve the original Date by copying before mutating.
const original = new Date(2026, 2, 1, 12, 30, 45, 100);
const adjusted = new Date(original);
adjusted.setMilliseconds(750);
console.log("Original ms:", original.getMilliseconds()); // 100
console.log("Adjusted ms:", adjusted.getMilliseconds()); // 750Setters always mutate their target. Clone first when other code still needs the old instant.
setMilliseconds(1500))Values above 999 roll into the next second.
const date = new Date(2026, 2, 1, 12, 30, 45, 0);
date.setMilliseconds(1500);
console.log(date.getSeconds()); // 46
console.log(date.getMilliseconds()); // 5001500 ms = 1 second + 500 ms. The engine normalizes instead of clamping to 999.
setMilliseconds(0) before second-level compare.performance.now() for benchmarks).setMilliseconds() Updates the ClockEngine loads the current local second and its fraction.
The milliseconds argument replaces the sub-second fraction.
Values ≥ 1000 or < 0 roll into adjacent seconds.
Date updates in place; epoch milliseconds are returned.
setMilliseconds() is 0–999 inside the current second—not epoch ms.end.getTime() - start.getTime().setUTCMilliseconds for UTC sub-second rules.Date.prototype.setMilliseconds() has been available since ES1. It works in every browser and Node.js.
Supported in Chrome, Firefox, Safari, Edge, Internet Explorer, and all Node.js versions. No polyfill required.
Bottom line: Safe everywhere. Remember sub-second range, mutation, and getTime() for duration.
setMilliseconds() writes the local sub-second fraction (0–999) on a JavaScript Date. Use it to adjust fractional seconds; use getTime() for measuring elapsed time.
Next, learn setMinutes() to change the minute component, or review getMilliseconds() for reading sub-seconds.
setMilliseconds(0) to drop sub-second noisesetHours(h, m, s, ms) for full time snapsgetMilliseconds() to verify resultsgetTime() for elapsed measurementsgetTime()Date for micro-benchmarksDate.setMilliseconds()Your foundation for local sub-second writes in JavaScript.
setMilliseconds(n)
Sub-second.
RangeUse getTime().
PitfallRolls seconds.
NormalizeKeep original.
PatternsetMilliseconds(1500) does not clamp at 999—it adds one second and sets milliseconds to 500. For elapsed time between two instants, subtract getTime() values instead.
Learn how to set the minute component (0–59) on a Date object in local time.
6 people found this page helpful