Timezoned unix timestamps

A JavaScript’s Date love/hate letter.

// TL;DR
const time = Math.floor(now.getTime() / 1000) + now.getTimezoneOffset() * 60;

Use case: I received a date from an external source (backend) in a UNIX timestamp format (number of seconds since January 1st, 1970, shortly named UTC). I wanted to know if that timestamp is in the future or in the past (so that I could validate a JWT token).

But the server’s time was in different timezone than mine. Which caused issues even though I was using epoch time:

const date: number = payload.date; // server's date
console.log(date); // 172863765
console.log(Date.now()); // 1728648454925

Long story short - regardless of what you do with your current date, you will still get UTC time including your timezone difference. Making both timestamps work withouth a timezone (GMT) required some manipulation:

const now = new Date();

// time elapsed since January 1st, 1970 in MILLISECONDS
const localTimeSinceEpoch = now.getTime(); // 1728648454925

// timezone difference IN MINUTES 🤯
const timezoneOffset = now.getTimezoneOffset(); // -180

// let's talk the same language - SECONDS
const timezonedTimestamp = Math.floor(localTimeSinceEpoch / 1000);
const offsetInSeconds = timezoneOffset * 60;
const timeSinceGMTEpoch = timezonedTimestamp + offsetInSeconds;

// everything's in seconds, let's compare
if (tokenExpiry < timeSinceGMTEpoch) {
  // not yet expired!
}