Why Meeting Times Across Time Zones Keep Going Wrong (DST and the UTC Trap)

You probably memorized that Seoul and New York are 14 hours apart. Then you check in July and it says 13. Where did the hour go?

Nothing is broken. A time difference is not a fixed number. This post covers when and why it changes, and where that bites you when scheduling.

1. A time difference is not a constant

A time difference is the gap between two UTC offsets. The catch is that those offsets do not stay the same all year.

In summer New York moves its clocks forward an hour. Seoul does not move at all, so the gap shrinks by one. Los Angeles swings the same way, between 17 and 16 hours.

2. No daylight saving in Korea means no instinct for it

South Korea does not observe daylight saving time. The last time it did was 1987–1988; since then it has stayed at UTC+9 year round.

If you only ever live on that side, you never experience clocks moving. A time difference feels like a constant you can memorize once. The fact that the other country shifts twice a year simply drops out of the mental model.

Tokyo shares UTC+9, so that gap really is a permanent zero — Japan has no DST either. Dubai (UTC+4) is fixed as well. Working mostly with those pairs makes the habit even stickier.

3. Countries switch on different dates, creating two odd windows a year

Here is the real trap. Countries that do observe DST do not switch on the same day.

• United States — starts the second Sunday in March, ends the first Sunday in November
• European Union — starts the last Sunday in March, ends the last Sunday in October

For 2026 that lands here:

So from March 8 to March 29 (about three weeks) and from October 25 to November 1 (about one week), only one of the two regions is on summer time. During those windows New York and London are 4 hours apart instead of the usual 5.

If you schedule a call with both US and European participants, those three weeks and that one week are a failure window that repeats every year.

4. The southern hemisphere moves the other way

Add Sydney and the arithmetic twists again. Southern hemisphere seasons are reversed, so its summer time period is reversed too.

• Sydney DST: first Sunday in October → first Sunday in April
• Standard (AEST) UTC+10 → summer (AEDT) UTC+11

The gap from Seoul swings between 1 and 2 hours. And note that October — when Sydney moves its clocks forward — is the same month Europe moves its clocks back. In the same week, one region springs forward while the other falls back.

5. Hardcode a time zone into a scheduler and the day slips too

People being confused is one thing. Automation with a hardcoded offset fails far more quietly.

Server schedulers (cron) usually evaluate expressions in UTC. Suppose you want a job at 07:30 Korean time:

KST 07:30 = 22:30 UTC, but on the previous day

If you change only the time to 30 22 and leave the day-of-week field alone, a weekday job starts running on Sunday night. 22:30 UTC is the following morning in Seoul, so the day field has to shift back by one as well — converting the hour and forgetting the day is a very common miss.

The worst part is that nothing errors out. The job just runs quietly on the wrong day, and you find out much later.

6. What to do instead

• Do not store fixed offsets like UTC+9 or GMT−5. Use region names (IANA time zones) such as Asia/Seoul or America/New_York, and daylight saving is applied for you. The world clock on this site works the same way, recalculating the offset for the current date.
• Put the local time in the invitation. Writing "4 PM KST (3 AM New York)" lets the other side check your math.
• Double-check meetings in March and late October. If the date falls in a transition window, last month's offset no longer holds.
• Recurring meetings shift on transition days. Calendars handle it by time zone, but it still means the other side's local hour moves by an hour — say so in advance.

To line them up right now, open the World Clock and put Seoul, New York, London, and Sydney side by side. If you need the gap between two dates instead, the D-Day Calculator is in the same family.

Summary

• A time difference is not a constant — it changes twice a year with DST (Seoul–New York swings 13 ↔ 14 hours)
• Korea has no DST, so there is no instinct that anything moves
• The US and Europe switch on different dates, creating a ~3 week window in March and a ~1 week window in late October
• Sydney is in the southern hemisphere and moves in the opposite season
• When converting to UTC for a scheduler, shift the day of week too
• Store IANA time zone names, not offsets