To convert a JavaScript Date to a Unix timestamp you can use either the Date.prototype.getTime() or the Date.prototype.valueOf() method on the Date object, both of which return the number of milliseconds (ms) since the Unix Epoch, then divide it by 1000 to get the number of seconds since the Unix Epoch.

Shorthand:

Math.floor(new Date().getTime()) / 1000;

// Or the equivalent:
// Math.floor(new Date().valueOf()) / 1000;

A reusable function:

function dateToUnixEpoch(date) {
	return Math.floor(date.getTime()) / 1000;
}

const ms = dateToUnixEpoch(new Date("Jan 1, 2000"));

TypeScript

Here is a TypeScript version of the same function:

function dateToUnixEpoch(date: Date): number {
	return Math.floor(date.getTime()) / 1000;
}

Converting back to a JavaScript Date

To convert a Unix Timestamp back to a JavaScript date, just multiply it by 1000 to get the number of milliseconds and then pass it to new Date(...):

const seconds = 1577865600;
const date = new Date(seconds * 1000);

Background of Unix Epoch timestamps

A common time format in the world of computer programming is the Unix Timestamp which is represented by the number of seconds since January 1st, 1970 at 00:00:00 UTC.

Using the Unix Epoch timestamp can be useful if you, for example, want to compare two dates to each other. For example, say you want to sort some items by their creation date, you could convert the dates to

Other names for this type of timestamp are “Unix Time”, “Epoch Time”, and “Posix Time”.

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