Learn about time zones

Where to start

If time zones are new to you, read How time zones work first: it explains why the world is not on one clock and what an offset from UTC is. The guide to GMT, UTC and IST then sorts out the three sets of letters that Indian readers meet most often. The other guides each answer one practical question: how to choose an hour for a call that suits several countries, why noon on the clock is not noon by the sun, why some countries are half an hour off from their neighbours, and what happens to the date in the middle of the Pacific Ocean.

Guides that use this site's own data

Two of the guides are built from the same data as the rest of the site, so their tables are worked out, not copied: the list of every place on a half-hour or quarter-hour offset, and the list of cities where the clock runs farthest ahead of or behind the sun. They are rebuilt whenever the time zone database changes. Rules in use: IANA time zone database 2026f.

Then try it yourself

The quickest way to understand a time difference is to see it. Open the converter, pick two places and look down the planner table, or open the India time tables for an hour by hour view.