Local time is what your wall clock says. It depends on which time zone you're in and, in many places, whether daylight saving time is currently in effect. Local time is convenient for everyday life — you want 9 a.m. to mean roughly the same experience of morning regardless of where on the planet you happen to be.

UTC — Coordinated Universal Time — is a single universal reference. No time zones, no seasonal shifts, no ambiguity. When two systems or two people in different countries need to agree on exactly when something happens, they use UTC.

The difference between the two is your offset, measured in hours east (+) or west (−) of UTC. Your offset can change with the seasons if your region observes daylight saving time. The hero above shows both clocks side by side and the offset between them.

Before the late 19th century, every city kept its own local time, roughly pegged to when the sun crossed the highest point overhead. That worked fine until trains and the telegraph made "what time is it over there?" a question people needed to answer dozens of times a day. Railway timetables in particular were a mess.

In October 1884, delegates from 25 nations gathered in Washington, D.C. for the International Meridian Conference and voted — 22 to 1, with France and Brazil abstaining and the Dominican Republic voting against — to adopt the meridian through the Royal Observatory at Greenwich, England as the world's prime meridian. From it, the globe was divided into 24 hour-wide zones. For the next several decades, Greenwich Mean Time (GMT) served as the de facto global reference. France didn't formally adopt the Greenwich standard until 1911.

GMT was tied to the actual rotation of the Earth, which turned out to be a problem: the Earth is a slightly wobbly, gradually slowing clock. By the mid-20th century, atomic clocks could measure time far more precisely than the planet itself could keep it.

The name UTC and the coordinated broadcast system behind it date to 1960. The modern form — atomic seconds, with occasional "leap seconds" inserted to keep UTC within 0.9 seconds of astronomical time — took effect on January 1, 1972. UTC is effectively International Atomic Time (TAI) minus a whole-number count of leap seconds, maintained by the Bureau International des Poids et Mesures (BIPM). As of 2026, that count stands at 37, and no leap second has been inserted since 2016 — all tracked by the International Earth Rotation and Reference Systems Service (IERS).

The abbreviation itself is a diplomatic compromise. The English form is Coordinated Universal Time (CUT); the French form is Temps Universel Coordonné (TUC). Neither language "wins" — UTC was chosen so both sides could point to the same letters and read their own language into them.

Leap seconds may be on the way out. In 2022, the 27th General Conference on Weights and Measures (CGPM) voted (Resolution 4) to effectively abolish the leap second by or before 2035, widening the allowable drift between UTC and the Earth's rotation. The motivation is practical: leap seconds are a recurring headache for computer systems, and in recent years the Earth has actually been spinning a touch fast, raising the strange possibility of a negative leap second.

UTC shows up everywhere that ambiguity is expensive:

• Aviation. Flight plans, clearances, weather reports (METARs, TAFs), and NOTAMs are all in UTC. A pilot flying across five time zones uses one clock.
• The military. UTC is spoken as "Zulu" in the NATO phonetic alphabet ("oh-nine-hundred Zulu"), so operations in different theaters can synchronize without converting.
• Maritime navigation. Ship's logs, GPS fixes, and astronomical almanacs use UTC.
• Astronomy and spaceflight. Observations, launch windows, and spacecraft commands all reference UTC — the Earth's rotation is the very thing an observation is usually trying to factor out.
• Computers. Databases, log files, distributed systems, TLS certificates, HTTP headers, JWT expirations. Internal storage is almost always UTC; local time is applied at the edge when something is rendered for a human.
• Science and international scheduling. Anytime a precise moment matters and the audience spans the globe.

The common thread: UTC is the time you use when the question "which time zone did you mean?" shouldn't even be askable.

Daylight saving time is the practice of shifting clocks forward in spring and back in fall so that evenings get more usable daylight during the warmer months. It's a good candidate for "most disliked widely-used system in modern life."

Origin

Benjamin Franklin is often credited, but that's a misread. His 1784 letter to the Journal de Paris was satirical — he proposed taxing window shutters and firing cannons at sunrise to wake Parisians earlier and save candles. It's a joke essay, not a policy proposal.

The first serious proposal came from George Hudson, a New Zealand entomologist, in 1895. Hudson wanted more evening daylight to collect insects after work, and pitched a two-hour seasonal shift to the Wellington Philosophical Society.

A decade later, British builder William Willett independently reached the same idea and had the tenacity to campaign for it. His 1907 pamphlet The Waste of Daylight argued for shifting clocks in 20-minute increments across four Sundays each spring. Willett lobbied Parliament for years and died in 1915, a year before Britain adopted it.

Adoption

The push that finally got DST implemented wasn't about evening leisure — it was about coal. On April 30, 1916, during the First World War, Germany and Austria-Hungary became the first countries to adopt DST nationally as a wartime fuel-conservation measure. Britain followed on May 21, 1916. The United States got there on March 19, 1918, with the Standard Time Act — a single piece of legislation that also formalized U.S. time zones.

U.S. DST was then repealed the following year, returned during WWII as "War Time," disappeared again, and bounced around as a patchwork state and local option until the Uniform Time Act of 1966 standardized it nationally. States are allowed to opt out; Arizona (except the Navajo Nation) and Hawaii do. States can't unilaterally stay on permanent DST — that takes an act of Congress.

The modern debate

The energy-savings case that originally justified DST has mostly evaporated in a world of air conditioning and LED lighting, and a long line of studies has linked the biannual clock change to spikes in heart attacks, car accidents, and workplace injuries in the days immediately afterward. Most people asked in polls want the changes to stop. What's disputed is which clock to stop on: standard time year-round (favored by sleep researchers) or DST year-round (favored by evening-daylight fans).

In March 2019, the European Parliament voted to abolish mandatory seasonal clock changes in the EU, originally targeting 2021. The Council of the EU never agreed on how to implement it and, as of 2026, European clocks still change twice a year.

In the United States, the Sunshine Protection Act — which would make DST permanent year-round — passed the Senate by unanimous consent in March 2022, then died in the House. It has been reintroduced in subsequent sessions but has not passed.

For now, twice a year, a large fraction of the planet sets its clocks to a number the Earth itself disagrees with.