Guides
Short, practical explanations of the things about time that break software and meetings. No tool required — these are here to be read.
Every tool on this site carries its own methodology, but some ideas are bigger than any one tool and keep causing the same mistakes. These pages cover those. They explain mechanisms rather than listing dates, because mechanisms stay true and dated rules do not.
- UTC and GMT are not the same thing One is a standard that never moves; the other is a zone that springs forward. The same zone reports GMT in January and GMT+01:00 in July.
- A time zone is not an offset An offset belongs to a place at an instant, not to a place. Storing the number instead of the zone name is how future dates go quietly wrong.
- The hour that does not exist, and the hour that happens twice A clock change deletes one local hour and repeats another. One of those errors is loud; the other is silent, which makes it the expensive one.
- Why some places are 45 minutes out Whole-hour offsets are a convention, not a rule. Rounding a half-hour zone produces an answer that looks exactly like a meeting time.
Where to go next
If you want the specific, sourced record of what has actually changed in the time zone database and where, that is time zone changes. If you want to know how the tools are tested and what we measure, that is how we test. The data behind the place pickers, its versions and what we altered, is on where our data comes from.