JavaScript Dates
A timestamp is the number of milliseconds since the start of 1970.
That moment is called the Unix epoch.
Storing a timestamp avoids every timezone and formatting question.
Example
javascript
const timestamp = Date.now();
console.log(typeof timestamp);
console.log(timestamp > 0);The value changes constantly, so the checks are what matter.
Date.now() gives the current timestamp.
date.getTime() gives the timestamp of a date.
new Date(timestamp) converts one back.
Syntax
javascript
Date.now();
new Date(timestamp);JavaScript uses milliseconds; many other systems use seconds.
A timestamp and a Date hold exactly the same information.
Example
javascript
const date = new Date(Date.UTC(2026, 0, 15));
const timestamp = date.getTime();
const back = new Date(timestamp);
console.log(back.getUTCFullYear());
console.log(back.getTime() === timestamp);Nothing is lost in the round trip.
Timestamp zero is the very start of 1970, in UTC.
Example
javascript
const epoch = new Date(0);
console.log(epoch.getUTCFullYear());
console.log(epoch.getUTCMonth());
console.log(epoch.getUTCDate());The output is 1970, 0, 1.
Many APIs send seconds, not milliseconds.
Multiplying by 1000 is the fix, and forgetting it lands you in 1970.
Example
javascript
const seconds = 1767225600;
const wrong = new Date(seconds);
const right = new Date(seconds * 1000);
console.log(wrong.getUTCFullYear());
console.log(right.getUTCFullYear());The first is barely past the epoch; the second is the intended date.
Subtracting two timestamps gives the duration.
Example
javascript
const started = Date.now();
let total = 0;
for (let i = 0; i < 100000; i++) {
total += i;
}
const elapsed = Date.now() - started;
console.log(elapsed >= 0);
console.log(typeof elapsed);performance.now() is more precise for timing code.
Numbers sort reliably, which makes timestamps ideal for ordering.
Example
javascript
const events = [
{ name: "second", at: Date.UTC(2026, 0, 15) },
{ name: "first", at: Date.UTC(2026, 0, 1) }
];
const sorted = events.slice().sort(function (a, b) {
return a.at - b.at;
});
console.log(sorted.map(function (e) { return e.name; }).join(","));The output is first,second.
Complete Example
html
<!DOCTYPE html>
<html>
<head>
<title>JavaScript Timestamps</title>
</head>
<body>
<h1>Timestamps</h1>
<p id="out"></p>
<script>
const now = Date.now();
const epoch = new Date(0);
const date = new Date(Date.UTC(2026, 0, 15));
document.getElementById("out").innerHTML =
"Now: " + now +
"<br>Epoch year: " + epoch.getUTCFullYear() +
"<br>Fixed date timestamp: " + date.getTime() +
"<br>Back to a date: " + new Date(date.getTime()).toISOString().slice(0, 10);
</script>
</body>
</html>Run the above example in the Try It Editor.
Try the seconds trap:
Divide the timestamp by 1000 and see the date jump back to 1970.
Date.now() gives the current one.new Date(timestamp) converts it back.Storing a timestamp sidesteps timezone and format questions entirely.
The seconds-versus-milliseconds mix-up is the classic bug here.
Convert to a Date only at the moment you need to display it.