Building a World Clock with Automatic DST — The Timezone Pitfalls

Timezone handling is where confident developers become humbled developers. Here's what I learned building a 33-city world clock.

The Wrong Way

// DON'T do this
function getTimeInNewYork(date) {
  const offset = -5; // EST
  return new Date(date.getTime() + offset * 60 * 60 * 1000);
}

This breaks during Daylight Saving Time. New York is UTC-4 in summer, UTC-5 in winter.

The Right Way: Intl.DateTimeFormat

function getTimeInZone(timezone) {
  return new Intl.DateTimeFormat('en-US', {
    timeZone: timezone,
    hour: '2-digit',
    minute: '2-digit',
    second: '2-digit',
    hour12: false,
  }).format(new Date());
}

getTimeInZone('America/New_York'); // Automatically handles DST

Intl.DateTimeFormat uses the IANA timezone database, which includes all historical and current DST rules.

Getting UTC Offset

function getUtcOffset(timezone) {
  const date = new Date();
  const utcDate = new Date(date.toLocaleString('en-US', { timeZone: 'UTC' }));
  const tzDate = new Date(date.toLocaleString('en-US', { timeZone: timezone }));
  const diff = (tzDate - utcDate) / (1000 * 60 * 60);
  return diff; // e.g., 9 for Seoul, -4 or -5 for New York
}

Is It Day or Night?

function isDaytime(timezone) {
  const hour = parseInt(
    new Intl.DateTimeFormat('en-US', {
      timeZone: timezone,
      hour: 'numeric',
      hour12: false,
    }).format(new Date())
  );
  return hour >= 6 && hour < 20;
}

Real-Time Updates

Use setInterval to update all clocks, but be careful about performance:

function startClock(timezones, onTick) {
  const tick = () => {
    const now = new Date();
    const times = timezones.map(tz => ({
      timezone: tz,
      time: getTimeInZone(tz),
      offset: getUtcOffset(tz),
      isDay: isDaytime(tz),
    }));
    onTick(times);
  };

  tick(); // Immediate first render
  return setInterval(tick, 1000);
}

The 33 Cities

I chose cities to cover all major business hubs across every region. The key insight: use IANA names, not abbreviations. America/New_York works forever; EST is ambiguous and breaks.

Live at toolzip.app/tools/world-clock — 33 cities, DST-aware, real-time.