Why Time Differences Between Cities Change During the Year
A time difference that changes with the season isn’t a bug — it happens whenever one city observes daylight saving time and the other doesn’t (or observes it on a different schedule).
It’s tempting to memorize a single number — "Bangkok is 7 hours ahead of London" — and treat it as always true. But if either city observes daylight saving time, that number can shift by an hour for part of the year.
A real example: Bangkok and London
Bangkok (Asia/Bangkok) does not observe daylight saving time — it stays at UTC+7 all year. London (Europe/London) does observe it: UTC+0 in winter, UTC+1 during British Summer Time. That means:
- During UK winter (UTC+0): Bangkok is 7 hours ahead of London.
- During UK summer / BST (UTC+1): Bangkok is only 6 hours ahead of London.
Nothing about Bangkok changed — the shift is entirely because London moved. This same pattern applies to any pair of cities where one observes DST and the other doesn’t, or where they observe it on different calendar rules (see how DST works for the details).
This is exactly why Timemazone’s city-difference pages always compute the current difference live from each city’s real time zone, and flag it explicitly when the gap is about to change — instead of printing one fixed number as if it were true all year.