The knowledge of time
Everything time
means.
From the 9,192,631,770 atomic oscillations in one second to the political decision behind a time-zone boundary.
Time zones
What do PDT, MDT, CDT and EDT mean?
The four daylight-time abbreviations used across the contiguous United States, their UTC offsets and their standard-time partners.
Read the answer →How do time zones work?
Time zones convert one global instant into the local civil time chosen by each jurisdiction.
Read the answer →What is daylight saving time?
Why clocks move, why the date differs by country and why a city’s offset cannot be assumed year-round.
Read the answer →How does the International Date Line work?
The place where neighbouring civil time zones can carry dates one calendar day apart.
Read the answer →Why are time-zone borders crooked?
The textbook picture of 24 straight strips is a useful starting point. Real civil time follows communities, borders, trade and law.
Read the answer →Why do some time zones have 30- or 45-minute offsets?
Half-hour and quarter-hour offsets are reminders that civil time is chosen for people, not forced into a 24-box diagram.
Read the answer →Why does China have one time zone?
China spans several ideal longitude bands but uses one official national clock: China Standard Time, UTC+8.
Read the answer →Why do governments change time-zone rules?
Civil time is law and policy. A government can change an offset, daylight-saving schedule or boundary when it decides the practical benefits outweigh the disruption.
Read the answer →Why are time-zone abbreviations so ambiguous?
CST, IST and BST look concise but can name different places and rules. A city or IANA zone is safer.
Read the answer →Why Arizona and the Navajo Nation use different summer clocks
Most of Arizona stays on Mountain Standard Time all year, while the Navajo Nation observes daylight saving time: one of North America’s clearest examples of civil time following jurisdiction rather than a state outline.
Read the answer →Why India is UTC+5:30 and Nepal is UTC+5:45
India and Nepal show why whole-hour offsets are not a rule. Their national clocks are fifteen minutes apart, despite sharing a long border and broadly similar longitude.
Read the answer →Russia and Türkiye: when a government changes the clock permanently
Time-zone rules can change in a single legal decision. Türkiye’s 2016 move to permanent UTC+3 and Russia’s many regional changes show why future calendar data needs named zones and regular updates.
Read the answer →Why Australia has state-by-state daylight-saving time
Australia is not one clock. Its states and territories use several standard offsets and make separate choices about daylight saving, creating summer differences inside one country.
Read the answer →How time works
Why is a day 24 hours long?
Earth’s rotation gives us the day; ancient counting systems gave it 24 named hours.
Read the answer →Why are there 60 minutes in an hour?
The answer runs through Babylonian base-60 mathematics, Greek astronomy and medieval clockmaking.
Read the answer →Solar time vs clock time
Why the Sun is not always highest at 12:00 and why sunrise changes with both longitude and the calendar.
Read the answer →Standards
How does an atomic clock work?
Atomic clocks turn an invariant atomic transition into the world’s most stable tick.
Read the answer →UTC vs GMT: what is the difference?
GMT is a historic mean-solar reference and modern time-zone label; UTC is the atomic international time standard.
Read the answer →What is a leap second?
A one-second adjustment that has kept atomic civil time close to Earth’s irregular rotation.
Read the answer →IANA time zones vs UTC offsets: what is the difference?
UTC+1 is a number. Europe/Paris is a history, a rule set and a prediction for a particular place.
Read the answer →History
A short history of timekeeping
From shadows and water flow to pendulums, quartz, atoms and globally synchronised networks.
Read the answer →How do calendars measure time?
Days follow Earth’s rotation, months echo lunar cycles and years track Earth’s orbit—then rules make them fit.
Read the answer →Who invented time zones?
Standard time was not invented by one person. Railway planners, scientists and public institutions converged on a shared answer to a very practical problem.
Read the answer →What did the 1884 International Meridian Conference decide?
The Washington conference selected Greenwich as a common zero meridian. It did not impose every modern time-zone border on the world.
Read the answer →The International Date Line: history, rules and exceptions
The Date Line separates consecutive calendar dates, but it is a convention that bends around political choices rather than a legal wall at exactly 180 degrees.
Read the answer →British and American daylight saving time: a short history
Britain and the United States both move clocks seasonally, but their schedules and legal histories are different—and that difference matters for transatlantic meetings.
Read the answer →Why Samoa skipped a whole day in 2011
Samoa moved across the International Date Line at the end of 29 December 2011, so Friday 30 December never occurred there. It is a vivid demonstration that civil dates are a policy choice.
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