시간대 변환 방법: 완전 가이드
시간대 간 시간 변환, UTC와 GMT의 차이.
What are time zones and how they work
The world is divided into 24 main time zones, each representing 1 hour from UTC. In practice, 38+ zones exist due to fractional offsets (India: UTC+5:30, Nepal: UTC+5:45).
UTC vs GMT: Practically identical. UTC is the modern scientific standard (atomic clocks). GMT is historical (Earth's rotation at Greenwich). Max difference: 0.9 seconds.
Convert with the NexTools timezone converter.
Main time zones table
| Zone | UTC Offset | Major cities |
|---|---|---|
| PST/PDT | -8/-7 | LA, SF, Vancouver |
| CST/CDT | -6/-5 | Chicago, Mexico City, Houston |
| EST/EDT | -5/-4 | New York, Miami, Toronto, Bogota |
| GMT/BST | 0/+1 | London, Dublin, Lisbon |
| CET/CEST | +1/+2 | Paris, Berlin, Madrid, Rome |
| IST | +5:30 | Mumbai, Delhi, Bangalore |
| CST | +8 | Beijing, Shanghai, Hong Kong |
| JST | +9 | Tokyo, Seoul |
| AEST/AEDT | +10/+11 | Sydney, Melbourne |
For international currency work, use the currency converter.
Daylight Saving Time: the complication that changes hours
DST users: USA (except AZ, HI), Canada, EU, UK, Australia (except QLD), NZ.
No DST: Mexico (abolished 2022), Japan, China, India, most of Africa/Asia/LATAM.
The chaos: Europe changes last Sunday March/October. US changes second Sunday March, first Sunday November. Dates differ, causing weeks where time differences shift.
How to schedule meetings across time zones
Overlap rule: For 3+ zones, find hours reasonable (8AM-8PM) for all. Example: NY + London + Mumbai:
- 9 AM NY = 2 PM London = 7:30 PM Mumbai ✓
- 7 AM NY = 12 PM London = 5:30 PM Mumbai ✓
Tip: Rotate times for recurring meetings.
In calendar invites: Always specify timezone. Google Calendar auto-converts.
Calculate exact differences with the NexTools converter.
Time zones in programming: best practices
Rule #1: Always store in UTC.
Rule #2: Use libraries. JS: Luxon or date-fns-tz. Python: zoneinfo. Java: java.time.ZonedDateTime.
Rule #3: Save user's timezone. Detect with Intl.DateTimeFormat().resolvedOptions().timeZone.
Rule #4: Run cron jobs in UTC. A 2:30 AM cron can run 0 or 2 times on DST change day.
See our Unix timestamp guide for more on dates in programming.
Special cases and timezone curiosities
India: UTC+5:30. One zone for a massive country (national unity).
China: UTC+8 for ALL of China, though it spans 5 zones geographically.
Nepal: UTC+5:45. Only 45-minute offset zone. Deliberately different from India.
Kiribati: UTC+14. First place where each new day starts.
Date Line: Samoa jumped from UTC-11 to UTC+13 in 2011, skipping December 30.
Timezone abbreviations: why they're confusing
Abbreviations are ambiguous:
- CST: Central (US, -6), China (+8), Cuba (-5)
- IST: India (+5:30), Ireland (+1), Israel (+2)
- EST: Eastern US (-5) or Eastern Australia (+10)
Solution: Use IANA identifiers: "America/New_York", "Asia/Kolkata". Unique and unambiguous.
For date calculations, use the date difference calculator.
Tools for working with time zones
NexTools: Timezone converter for quick conversions.
World Time Buddy: Visualize multiple zones in parallel.
timeanddate.com: Complete reference with DST and events.
In code: Luxon (JS), pytz/zoneinfo (Python), java.time (Java 8+).
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What is the difference between UTC and GMT
Practically none for daily use. UTC is the modern scientific standard (atomic clocks). GMT is historical. Max difference: 0.9 seconds. Used interchangeably.
Why do some countries have 30 or 45 minute timezone offsets
Political decisions. India chose +5:30 as compromise. Nepal chose +5:45 to differ from India. These are valid but complicate calculations.
Does DST change worldwide at the same time
No. US in March/November, Europe March/October, Australia April/October (opposite hemisphere). Many countries never change. This causes time differences to shift multiple times yearly.
How do I avoid timezone errors in my application
Store everything UTC. Convert to local only for display. Use libraries (Luxon, pytz). Detect user zone with Intl API. Run crons in UTC.
Which two time zones have the biggest difference
UTC-12 and UTC+14 = 26 hours apart. In inhabited zones, UTC-11 (American Samoa) and UTC+12 (Fiji) = 23 hours.
Why can CST mean 3 different things
Timezone abbreviations aren't formally standardized. Multiple zones share letters. Solution: use IANA identifiers (America/Chicago, Asia/Shanghai) which are unique.