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Convert times across 24+ time zones, understand UTC offsets, daylight saving rules, and schedule across continents.
24
Standard time zones
UTC±
Offset from Greenwich
DST
Daylight saving shifts
IANA
Official tz database
A time zone is a geographic region that observes a uniform standard time. The Earth rotates 360° in 24 hours, so each 15° of longitude corresponds to a 1-hour time difference. Most time zones are offset from Coordinated Universal Time (UTC) by whole hours, though some use 30- or 45-minute offsets (India: UTC+5:30, Nepal: UTC+5:45).
Before standardized time zones, every city kept its own local solar time. The growth of railroads in the 1800s made this chaotic — a train traveling from Washington to San Francisco would cross dozens of local times. In 1883, US railroads adopted four standard time zones, and in 1884, the International Meridian Conference established the global system based on Greenwich, England.
Daylight Saving Time (DST) shifts clocks forward 1 hour in spring ("spring forward") and back in fall ("fall back"). About 70 countries observe DST, but rules vary — the US and EU change on different dates. Arizona, Hawaii, and most of the tropics do not observe DST. The IANA tz database tracks all historical and current rules.
| Abbrev | Name | UTC Offset | Major Cities | DST? |
|---|---|---|---|---|
| EST/EDT | Eastern | −5 / −4 | New York, Toronto, Miami | Yes (Mar–Nov) |
| CST/CDT | Central | −6 / −5 | Chicago, Houston, Mexico City | Yes |
| MST/MDT | Mountain | −7 / −6 | Denver, Phoenix (no DST) | Arizona: No |
| PST/PDT | Pacific | −8 / −7 | Los Angeles, Seattle, Vancouver | Yes |
| GMT/BST | Greenwich | 0 / +1 | London, Dublin, Lisbon | Yes (Mar–Oct) |
| CET/CEST | Central European | +1 / +2 | Paris, Berlin, Madrid | Yes |
| IST | India Standard | +5:30 | Mumbai, Delhi, Bangalore | No |
| CST (China) | China Standard | +8 | Beijing, Shanghai, Taipei | No |
| JST | Japan Standard | +9 | Tokyo, Seoul, Osaka | No |
| AEST/AEDT | Aus Eastern | +10 / +11 | Sydney, Melbourne | Yes (Oct–Apr) |
Target Time = Source Time + (Target UTC offset) − (Source UTC offset) Example: 3:00 PM EST → IST EST = UTC−5, IST = UTC+5:30 Difference = +5:30 − (−5) = +10:30 3:00 PM + 10:30 = 1:30 AM (next day) Example: 9:00 AM PST → GMT PST = UTC−8, GMT = UTC+0 Difference = 0 − (−8) = +8 9:00 AM + 8 = 5:00 PM same day Example: 2:00 PM JST → EST JST = UTC+9, EST = UTC−5 Difference = −5 − 9 = −14 2:00 PM − 14 = 12:00 AM same day
To convert: subtract source offset from the time (getting UTC), then add target offset. The formula above combines both steps.
Check if DST is active in BOTH zones: US DST: 2nd Sunday March → 1st Sunday Nov Spring forward: 2:00 AM → 3:00 AM Fall back: 2:00 AM → 1:00 AM EU DST: Last Sunday March → Last Sunday Oct Spring forward: 1:00 AM UTC → 2:00 AM UTC Fall back: 1:00 AM UTC → 12:00 AM UTC The 'gap' weeks: Mar 10–31: US observes DST, EU does not Oct 27–Nov 3: EU ended DST, US has not During gap weeks: NYC↔London = 4 hours (not usual 5) NYC↔London = 6 hours (not usual 5) Always use IANA tz database for accuracy!
DST transitions cause 2-3 weeks per year where offsets between zones change. Scheduling across US/EU during these gap weeks is a common source of errors.
| Time Zones | Offset | 9–5 Overlap | Best Meeting Time |
|---|---|---|---|
| New York ↔ London | 5 hrs | 2–5 PM NYC / 7–10 PM LON | 10 AM NYC / 3 PM LON |
| New York ↔ Los Angeles | 3 hrs | 12–5 PM NYC / 9 AM–2 PM LA | 1 PM NYC / 10 AM LA |
| New York ↔ Mumbai | 10.5 hrs | 8:30–10 AM NYC / 7–8:30 PM MUM | 9 AM NYC / 7:30 PM MUM |
| London ↔ Tokyo | 9 hrs | 8–9 AM LON / 5–6 PM TYO | 8 AM LON / 5 PM TYO |
| San Francisco ↔ Berlin | 9 hrs | 8–9 AM SF / 5–6 PM BER | 8 AM SF / 5 PM BER |
| Sydney ↔ New York | 16 hrs | 6–9 AM SYD / 2–5 PM NYC | 8 AM SYD / 4 PM NYC prior day |
King Charles II established the Royal Observatory at Greenwich, England. The Prime Meridian (0° longitude) was eventually fixed here, making Greenwich Mean Time (GMT) the world's reference point for timekeeping.
US and Canadian railroads adopted four standard time zones (Eastern, Central, Mountain, Pacific) to replace the chaos of hundreds of local times. Before this, a train ride from Washington to San Francisco crossed over 200 local time standards.
Twenty-five nations met in Washington, DC and agreed on Greenwich as the Prime Meridian and the basis for 24 global time zones. This established the framework still used today, though many boundaries have been adjusted for political reasons.
Germany and Austria-Hungary became the first countries to implement DST during World War I to conserve coal. The UK, US, and others followed. Benjamin Franklin had suggested a similar concept in 1784, though his letter was satirical.
Coordinated Universal Time (UTC) was adopted as the world's time standard, based on atomic clocks rather than astronomical observation. Unlike GMT, UTC is defined precisely and adjusted with leap seconds to stay within 0.9 seconds of solar time.
Arthur David Olson created the tz database (also called the Olson database), which tracks every time zone rule change globally. It's used by Linux, macOS, Java, PHP, and virtually all software that handles time zones. Updated multiple times per year.
IANA — Time Zone Database
The IANA tz database contains every documented time zone rule from 1970 to present, covering ~600 zones. It's maintained by volunteers and updated several times annually. Every major operating system and programming language relies on it for accurate time zone conversions.
NIST — Official US Time
The National Institute of Standards and Technology maintains the US cesium atomic clock ensemble. NIST-F2 is accurate to 1 second in 300 million years. Time signals are broadcast via radio (WWV/WWVH) and internet (NTP) to synchronize clocks nationwide.
Daan & Aschoff (1975) — Circadian Rhythms
Research on circadian rhythms shows the human body clock runs on approximately a 24.2-hour cycle. Crossing time zones disrupts this cycle (jet lag), with recovery taking roughly 1 day per time zone crossed eastward and 1.5 days per zone westward. Light exposure is the primary resetting signal.
Kotchen & Grant (2011) — Energy Policy
Indiana's adoption of DST in 2006 provided a natural experiment. Researchers found DST increased residential electricity use by 1-4% due to higher heating and cooling costs, despite reduced lighting needs. The energy-saving rationale for DST is increasingly questioned worldwide.
There are exactly 24 time zones.
There are actually 37+ time zones in use. Several use half-hour offsets (India UTC+5:30, Iran UTC+3:30, Newfoundland UTC−3:30) and Nepal uses UTC+5:45. Some Pacific islands are UTC+13 or +14, overlapping with UTC−11/−10 on the previous day.
DST saves energy and was created for farmers.
DST was created during WWI to save coal for the war effort, not for agriculture. Farmers actually opposed it. Modern studies (Indiana 2006) show DST may increase energy use due to higher air conditioning demand. Health studies link DST transitions to increased heart attacks and traffic accidents.
China has one time zone, so it's always the same time everywhere.
China officially uses only UTC+8 (Beijing Time) despite spanning 5 geographic time zones. This means sunrise in far-western Xinjiang can be as late as 10:00 AM Beijing Time. Some residents use unofficial local time (Ürümqi Time, UTC+6) for daily activities.
The International Date Line is straight.
The International Date Line (IDL) zigzags dramatically to keep island nations and territories on the same day as their trading partners. In 2011, Samoa skipped December 30th entirely, jumping from UTC−11 to UTC+13 to align with Australia and New Zealand for business.
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