Learn about time

Clear, sourced answers to the questions people ask most about time, time zones and clocks.

• How is a second defined?One second is 9,192,631,770 periods of the radiation from the cesium-133 atom. The official SI definition, how it was measured before 1967, and why it may change around 2030.
• UTC vs GMTUTC and GMT show the same time on the clock. UTC is the atomic time standard the world sets clocks by; GMT is the UK's winter time zone and an older astronomical standard. Here's the difference.
• Leap secondsA leap second is an extra second added to UTC to keep atomic time in step with Earth's rotation. All 27 leap seconds since 1972, how they work, and the 2035 decision to stop them.
• International Date LineThe International Date Line runs roughly along 180° longitude in the Pacific. Cross it westward and you skip ahead a day. Why it zigzags, Kiribati's 1995 change, and Samoa's missing day.
• Why Nepal is UTC+5:45Nepal is 5 hours 45 minutes ahead of UTC, the only country whose national time is offset by 45 minutes. Why it chose +5:45, when it changed, and other odd time zones.
• Strange time zonesNepal's UTC+5:45, Kiribati's UTC+14, Samoa's skipped day, China's single zone, Spain's misplaced clocks and more. A sourced table of time zone anomalies you can download.
• Who invented time zones?Sandford Fleming usually gets the credit, but time zones had several inventors: the Great Western Railway, Quirico Filopanti, Charles Dowd, Fleming and William F. Allen. Here's who did what.
• Words about timeA list of words related to time, from punctual and tardy to ephemeral, perennial, diurnal and anachronism, with definitions, word origins and the stories behind time idioms.
• What punctual meansWhat does punctual mean? Definition, pronunciation, word origin (from Latin punctum, 'point'), synonyms and opposites, example sentences, punctuality around the world, and tips for being on time.
• Why we check the timeWhy do people check the time so often? Your body has a 24-hour clock, your mind hates uncertainty, and society runs on shared time. A sourced long read, from notched bones to atomic clocks.
• Who invented the watchWho invented the watch, the wristwatch, quartz, digital and smartwatches? A sourced timeline of 50 breakthroughs from Peter Henlein (c. 1505) to Rolex, Seiko, Casio and Apple.
• How time zones workWhy there are 24 zones, UTC, the date line and odd offsets.
• Keeping Time: the history of timeTen chapters from bone calendars to atomic clocks.