Coordinated Universal Time (UTC)

Coordinated Universal Time (UTC) is the world's reference time scale, kept by a network of atomic clocks and used as the basis for civil time in every country. Local time zones are expressed as offsets from it, such as UTC+5 for Pakistan or UTC−5 for New York in winter. Banks, payment systems, exchanges, computers and blockchains use UTC to time-stamp events so that everyone agrees on when something happened.

UTC is calculated by the International Bureau of Weights and Measures (BIPM) near Paris from clocks held in national laboratories around the world. It replaced Greenwich Mean Time (GMT) as the formal standard, although the two are within a second of each other and are often used interchangeably. The abbreviation is a compromise between the English "CUT" and French "TUC". In crypto, UTC is the default clock for daily price candles, funding payments and many settlement cut-offs.

Imagine a group of friends in different cities planning a video call. Saying "let's meet at 3 o'clock" is useless, because 3 o'clock means something different in each place. If they all agree on one shared reference clock and each works out their own local time from it, nobody turns up early or late.

UTC is that shared clock for the whole planet. It does not move with summer time and does not belong to any one country, which makes it the natural choice for systems such as stock exchanges and blockchains that never sleep and serve users everywhere.

UTC combines two kinds of time:

• Atomic time. International Atomic Time (TAI) is a weighted average of hundreds of atomic clocks in dozens of national laboratories, giving an extremely steady count of seconds.
• Earth-rotation time. UT1 tracks the actual rotation of the Earth, which is slightly irregular.

Since 1972, UTC has ticked at the atomic rate but has been kept within 0.9 seconds of UT1 by occasionally inserting a "leap second", announced by the International Earth Rotation and Reference Systems Service (IERS). Twenty-seven leap seconds have been added, the latest on 31 December 2016, so the gap is:

• $\text{TAI}$ is International Atomic Time.
• $\text{UTC}$ is Coordinated Universal Time.
• $10\,\text{s}$ is the offset set when leap seconds began in 1972, and $27\,\text{s}$ is the total of leap seconds added since.

For example, when an atomic clock running on TAI reads 12:00:37, UTC reads 12:00:00.

Leap seconds have caused real outages because software often does not expect a minute with 61 seconds. The leap second of 30 June 2012 triggered failures at several websites and airline booking systems, and a bug at Cloudflare on 1 January 2017 disrupted part of its DNS service. In November 2022 the General Conference on Weights and Measures (CGPM) resolved that the permitted difference between UT1 and UTC should be increased by 2035. The details of how and when the new rule takes effect were left for later decisions.

Traditional markets keep local hours, but much of their infrastructure runs on UTC behind the scenes. SWIFT messages and many RTGS systems record times in UTC or alongside it, and CLS coordinates its foreign-exchange settlement window across central banks in several time zones. European rules for trading venues require clocks to be synchronised to UTC so that trades can be reconstructed. A bank's cut-off time and a payment's value date are usually stated in local time, so converting to UTC avoids missed deadlines.

Crypto markets trade around the clock with no closing bell, so they need an agreed point to start and end each "day":

• Daily candles. Most major crypto exchanges and data sites open and close the daily candle at 00:00 UTC, which defines the daily open and close, trading volume and percentage change.
• Funding payments. On many perpetual futures markets the funding rate is exchanged at fixed UTC times, commonly every eight hours from 00:00 UTC, although some contracts settle more often.
• Settlement and expiry. Exchange-listed crypto futures and options commonly expire at a published UTC time, and period figures such as year-to-date returns are usually counted in UTC.
• Block timestamps. Bitcoin and Ethereum record each block with a Unix timestamp, the number of seconds since 00:00:00 UTC on 1 January 1970, which ignores leap seconds.

Because a "daily" chart depends on when the day starts, prices quoted in different time zones can show different daily highs and lows for the same market. Analysts comparing figures such as an all-time high or a relative strength index reading should check that sources use the same UTC cut-off.

• BIPM: Time metrology
• International Earth Rotation and Reference Systems Service (IERS)