Time Difference Calculator

Time differences between cities are surprisingly hard to compute correctly because they depend on every region's daylight saving rules, half-hour offsets (India, Iran, Newfoundland), forty-five-minute offsets (Nepal, the Chatham Islands), and the occasional government decision to skip or repeat a day (Samoa famously skipped 30 December 2011). This calculator uses the IANA Time Zone Database, which encodes every such rule going back to 1880, to compute exact city-to-city offsets in real time.

The IANA TZDB was founded by Arthur David Olson at the National Institutes of Health in 1986 and is now maintained collaboratively at iana.org/time-zones. It powers the timezone APIs of every major operating system - Linux, macOS, iOS, Android, FreeBSD - plus the runtime of every major language: Java, Python, JavaScript (via the Intl API in browsers), Go, Rust, and PostgreSQL. When a government changes its DST rules, the maintainers publish a new release within days, and the change propagates worldwide within weeks.

Daylight Saving Time, which doubles the complexity of timezone math, was popularised by the New Zealand entomologist George Hudson in 1895 and by the English builder William Willett in 1907. Germany adopted DST on 30 April 1916 to save coal during the First World War, and the rest of Europe followed within months. The US adopted DST in 1918, repealed it in 1919, reintroduced War Time in 1942, made it permanent in 1966, and most recently extended it under the 2007 Energy Policy Act.

Around half the world's population now observes DST. The principal hold-outs are most of Africa, most of Asia (including India, China, Japan, Korea, the Gulf states, and most of Southeast Asia), most of South America (except Chile), and Hawaii plus most of Arizona in the United States. Within Europe, the EU voted in 2019 to abolish the seasonal shift but has not yet implemented the decision; the UK and EU still switch every March and October.

India is one of the more interesting timezone cases. The Indian Standard Time meridian at 82.5 degrees east, set at the Madras (now Chennai) Observatory in 1906, gives India a single year-round UTC+5:30 offset. The half-hour was a political compromise. India considered DST during the 1962 Sino-Indian war and the 1971 Bangladesh war and rejected it both times. The single Indian timezone for a country spanning 2,933 km east-to-west is unusual; for comparison, the US has six civilian zones for 4,500 km.

China is even more unusual. The Chinese government collapsed five historical zones into a single nationwide Beijing Time (UTC+8) in 1949, which means the sun rises in westernmost Xinjiang at around 10 AM in summer. Russia, by contrast, runs 11 separate timezones across its 9,000 km span and has flip-flopped on DST repeatedly (adopted permanently in 2011, then dropped in 2014, with the regions choosing standard or daylight as their year-round basis).

Half-hour and forty-five-minute offsets are minority cases worth knowing. India (+5:30), Sri Lanka (+5:30), Iran (+3:30), Afghanistan (+4:30), Nepal (+5:45), Myanmar (+6:30), the Cocos Islands (+6:30), the Marquesas (-9:30), Newfoundland (-3:30 / -2:30), the Australian states of South Australia and the Northern Territory (+9:30 / +10:30), and the Chatham Islands of New Zealand (+12:45 / +13:45) all break the every-hour rule. This calculator handles all of them correctly via the IANA TZDB.