Unix timestamps in JavaScript

Get the current Unix timestamp

const millis = Date.now();                    // 1700000000000
const seconds = Math.floor(Date.now() / 1000); // 1700000000

Convert a timestamp to a Date

const date = new Date(1700000000 * 1000); // seconds → ms first
date.toISOString(); // "2023-11-14T22:13:20.000Z"

new Date(1700000000) is 20 days after the 1970 epoch. If a timestamp is 10 digits, multiply by 1000.

Convert a Date to a timestamp

const millis = date.getTime();
const seconds = Math.floor(date.getTime() / 1000);

Parse an ISO 8601 string

const date = new Date("2023-11-14T22:13:20Z");
date.toLocaleString("en-US", { timeZone: "Asia/Tokyo" });

A date-only string like "2023-11-14" parses as midnight UTC, but "2023-11-14T00:00" (no zone) parses as local time. That's a genuine spec quirk.