Time zones are a classic source of bugs: meetings scheduled an hour off, reports that miss a day, jobs that run twice. Most of the pain comes from a small set of avoidable mistakes.
1. Storing local times without a zone
A value like 2026-03-08 02:30 is ambiguous. Store instants in UTC (or as an ISO 8601 string with an offset, such as 2026-03-08T02:30:00Z) and convert to local time only when displaying it.
2. Using abbreviations
CST can mean Central Standard Time, China Standard Time, or Cuba Standard Time. IST can be India, Ireland, or Israel. Use IANA names such as America/Chicago, Asia/Kolkata, or Europe/London, which are unambiguous and carry the daylight saving rules.
3. Confusing offsets with zones
An offset like UTC-5 is a fixed number. A zone like America/New_York is a set of rules that changes between UTC-5 and UTC-4 during the year. Recurring events need a zone, not just an offset.
4. Ignoring daylight saving transitions
- When clocks spring forward, a local hour doesn't exist (2:30 AM may be skipped).
- When clocks fall back, a local hour happens twice, so a local time can map to two different instants.
- Not all places observe DST, and the rules change by government decision.
5. Assuming whole-hour offsets
India is UTC+5:30, Nepal is UTC+5:45, and some regions use half-hour offsets. Code that assumes whole hours will be wrong for millions of people.
6. Adding 24 hours to mean 'tomorrow'
On a DST change a local day can be 23 or 25 hours long. Use a date library to add calendar days in a specific zone.
7. Scheduling recurring meetings in UTC
A meeting at 09:00 in New York stays at 09:00 local all year, which is a different UTC time in summer and winter. Store the zone and local time for recurring events.
Frequently asked questions
+What is the best way to store dates and times?
Store instants in UTC (or ISO 8601 with an offset) and convert to the user's time zone only when displaying.
+Why shouldn't I use EST or IST?
Abbreviations are ambiguous and don't encode daylight saving rules. Use IANA zone names like America/New_York.
+How do I convert between time zones?
Convert the local time to a UTC instant using the source zone's rules, then express that instant in the target zone.
+Which time zones use half-hour offsets?
Examples include India (UTC+5:30), and Nepal uses UTC+5:45.
Timezone Converter
Free, runs in your browser — nothing you enter is uploaded.