Fundamentals
Understanding Timezones: A Practical Guide
How timezones work, why offsets are not always whole hours, and how IANA timezone names keep software accurate.
· 6 min read
A timezone is a region that keeps the same standard time. The world is divided into roughly 38 of them, offset from UTC in steps that are usually — but not always — whole hours.
Why some offsets are 30 or 45 minutes
India runs on UTC+5:30, Nepal on UTC+5:45, and parts of Australia on UTC+9:30. These half- and quarter-hour offsets exist for geographic and political reasons, and they are a frequent source of scheduling bugs in software that assumes whole-hour offsets.
IANA timezone names
Software should never store "EST" or "+05:30". Instead it uses IANA names like America/New_York or Asia/Kolkata. These names encode the full history of a region’s clock rules, including daylight saving transitions, so a date in the past or future converts correctly.
- America/New_York — automatically switches between EST and EDT.
- Europe/London — switches between GMT and BST.
- Asia/Kolkata — fixed at UTC+5:30 with no DST.
The practical takeaway
When you schedule across borders, think in terms of cities, not abbreviations. "New York" is unambiguous; "EST" is not, because New York is on EDT for more than half the year.