Time zones are one of those topics that look simple until you ship something and a user in another country sees the wrong time. Here are the traps I keep seeing, and how to reason about them in 2026.

UTC is not a time zone, and GMT is not UTC

UTC (Coordinated Universal Time) is a time standard, not a region. GMT is a time zone that happens to share the same offset as UTC most of the year. For storage and math, always think in UTC. Treat GMT as just another named zone.

Rule 1: store timestamps in UTC

Store every instant as UTC (or an epoch value). Convert to a local zone only at the edges, when you display to a user. If you store local times, you will eventually lose the offset and never recover the true instant.

Rule 2: an offset is not a zone

+09:00 tells you the offset right now. It does not tell you the zone, because zones change offset across the year due to daylight saving time. Store the IANA zone name (like America/New_York), not just the offset. The offset is derived from the zone plus the date.

Rule 3: DST is where it hurts

The same wall-clock time can happen twice (fall back) or never (spring forward). Scheduling "9am every day" is a zone-aware operation, not an offset-aware one. Libraries like the built-in Intl.DateTimeFormat and Temporal (now widely available) handle this correctly if you give them a zone name.

new Intl.DateTimeFormat('en-US', {
  timeZone: 'Asia/Tokyo',
  dateStyle: 'short',
  timeStyle: 'short',
}).format(new Date());

Rule 4: scheduling across teams is an overlap problem

For a distributed team, the useful question is not "what time is it there" but "when do our working hours overlap". That is a set-intersection over each person's 9-to-5 expressed in UTC.

A tool for the human side

When I just need to eyeball overlaps and pick a meeting time without writing code, I use the free tool I built: ZonePlan, a time zone meeting planner and live world clock.

If you want the practical playbook for picking meeting times, I wrote it up here: How to schedule a meeting across time zones.

What is the worst time zone bug you have shipped? Mine involved a cron job that ran twice every November.