Current Time

Knowing the current time precisely is the most basic and most important function of any civil timekeeping system. From booking flights and trains to executing financial trades to coordinating satellite passes, the entire modern economy runs on the assumption that everyone agrees on what time it is. The page you are reading uses the browser's built-in Intl.DateTimeFormat API, which is itself synchronized to your device's system clock via Network Time Protocol (NTP) running against atomic-clock-anchored time servers.

The concept of standard civil time is newer than most people realise. Until 1840 every village ran on local solar noon, which made cross-country trade and travel a clerical nightmare. The Great Western Railway in the United Kingdom imposed ‘railway time’ - synchronized to Greenwich - across its network in 1840, and the rest of Britain followed by 1855. The United States held out longer: each city maintained its own clock until the Pennsylvania Railroad finally pushed Congress into the Standard Time Act of 1918.

Greenwich Mean Time (GMT) became the international reference under the International Meridian Conference of 1884, which placed the prime meridian at the Royal Observatory in Greenwich, London. The conference also created the worldwide system of one-hour timezones radiating east and west. The conference's 24 founding nations included France (which abstained, preferring the Paris meridian), and Spain, Brazil, and the Dominican Republic (which voted against).

Coordinated Universal Time (UTC) replaced GMT as the international reference in 1972. UTC is anchored to International Atomic Time (TAI) - a weighted average of around 400 caesium and rubidium atomic clocks maintained by the Bureau International des Poids et Mesures (BIPM) outside Paris. UTC differs from TAI by an integer number of seconds (the ‘leap seconds’) which are added or skipped to keep UTC within 0.9 seconds of the solar UT1. The latest leap second was inserted on 31 December 2016.

Indian Standard Time was established at the Madras (now Chennai) Observatory in 1906, anchored to the 82.5 degrees east meridian which runs through Mirzapur in Uttar Pradesh. The offset of UTC+5:30 was a political compromise placing India between UTC+5 (now Pakistan) and UTC+6 (now Bangladesh). India does not observe DST. The National Physical Laboratory in New Delhi now maintains the IST signal using a network of caesium-fountain clocks traceable to UTC.

Modern computers synchronize to UTC via NTP, which was invented by David Mills at the University of Delaware in 1985. NTP servers are organised in a hierarchical stratum: Stratum 0 are the atomic clocks themselves; Stratum 1 are the servers directly connected to them; Stratum 2 and beyond are the public NTP pool servers that the rest of the internet uses. A typical home computer is synchronized within 10 to 50 milliseconds of UTC at all times. A typical mobile phone within 1 second.

The IANA Time Zone Database, which translates UTC into the local civil time shown on this page, was founded by Arthur David Olson at the National Institutes of Health in 1986 and now contains the full rule history for roughly 600 timezones since 1880. Every modern operating system (Linux, macOS, iOS, Android, Windows) and every modern programming language bundles a copy. The browser Intl.DateTimeFormat API is the JavaScript binding to the system's TZDB.