Time zones are not fixed, and that is why invites slip

You worked out once that they are four hours behind. You have been scheduling on that ever since. Then one week the call lands an hour out, and nobody can see why.

Nothing was mistyped. The four hours stopped being four hours.

An offset is a reading, not a property

A place does not have a fixed distance from anywhere else. What it has is a UTC offset that is true on a particular date, and that offset moves — twice a year in the many places that observe daylight saving, and occasionally at other times when a government changes the rules outright.

So the difference between two clocks is a subtraction of two offsets, and it is only valid for the date those offsets were read.

Why the gap moves — the three cases

Say a place on +01:00 in part of the year shifts to +02:00 in another part. Pair it with somewhere else and watch what happens.

• The other place does not shift at all. Plenty of places do not. Against a fixed −03:00, the gap is 1 − (−3) = 4 hours for part of the year and 2 − (−3) = 5 hours for the rest. One hour of drift, twice a year.
• The other place shifts too, but on a different date. Then there is a window — days or weeks long — where one has moved and the other has not, and the gap is wrong by an hour for exactly that window before snapping back.
• The other place is in the southern hemisphere. Its summer is the other summer, so it shifts the opposite way. If it goes from −03:00 to −04:00 while the first goes from +01:00 to +02:00, the gap swings from 4 hours to 2 − (−4) = 6 hours. Two hours of movement, from one change each.

The transition dates themselves differ by country and are changed from time to time, so there is no rule of thumb here. The only reliable move is to ask each person what offset they will be on for the date of the meeting.

Not every offset is a whole number of hours

Half-hour and quarter-hour offsets are real and ordinary. A meeting at 15:00 UTC reads as 20:30 on a +05:30 clock and 20:45 on a +05:45 one. Rounding either to the nearest hour puts somebody in the wrong meeting.

The error that actually ruins calls: the day

Offsets in current use run from −12:00 to +14:00. That is a twenty-six-hour spread, which is wider than a day — so the same instant can genuinely fall on three different dates.

Worked, all checked:

• 15:00 UTC on Tuesday 15 September 2026 is 03:45 on a +12:45 clock — and it is Wednesday the 16th there.
• A meeting called for 23:30 on a −12:00 clock on Tuesday the 15th reads 01:30 on a +14:00 clock, on Thursday the 17th. Two calendar days later, same moment.
• Month ends do not protect you either. 23:00 UTC on Wednesday 30 September 2026 is 13:00 on 1 October for a +14:00 clock — a new month.
• Nor do leap years. 23:00 UTC on 29 February 2028, which is a Tuesday, is 13:00 on 1 March 2028 for the same clock.

The arithmetic is one sum — local = instant + their offset — but you have to carry the day, and carrying the day is what people forget.

Why the tool refuses to know about cities

The free Meeting Time Across Time Zones tool asks you to type each person's UTC offset rather than pick their city from a list, and that is deliberate.

A city list means shipping a copy of the world time-zone database, which is revised several times a year as governments change rules, sometimes with only weeks of notice. A copy baked into an offline tool starts being quietly wrong within months — and a scheduling tool that is confidently wrong is worse than one that asks. An offset is something anyone can read off their own phone in seconds.

So a zone abbreviation is rejected with a message rather than guessed at, and an unreadable offset is never silently read as zero, because that would produce a plausible answer that is wrong by hours.

What it gives you

Paste a name, an offset and the hours each person will meet, and you get the proposed time on every clock with the day it lands on marked, a plain list of who it inconveniences and by how many minutes, and every slot that fits inside everybody's hours. Windows that cross midnight are handled as night shifts, not errors. If nothing suits everyone it says so and shows the least-bad options with names attached, because the honest answer is sometimes that one person takes it early.

It does not apply daylight saving, and it says so loudly. Bring the offsets for the day of the meeting and the arithmetic is exact.