Why the world uses UTC as a shared clock

A meeting scheduled for 14:00 UTC identifies a shared reference time. Participants may see different local clock readings, but they can convert those readings to the same instant.

That distinction—between a common reference and local civil time—is the key to understanding Coordinated Universal Time, abbreviated UTC.

What UTC measures

UTC is an international reference time scale produced by the International Bureau of Weights and Measures, or BIPM. It runs at the same rate as International Atomic Time, known as TAI, but differs from it by a whole number of seconds.

Atomic time and Earth’s rotation are not identical ways of keeping time. Atomic clocks supply a highly stable basis for measuring seconds; the Earth does not rotate at a perfectly uniform rate. The time scale called UT1 tracks that rotation.

Under the current system, leap seconds keep UTC within 0.9 seconds of UT1. NIST’s leap-second explanation describes the relationship. UTC is therefore not simply another name for Earth-rotation time.

Local clocks apply offsets

Local time is expressed in relation to UTC using an offset. In a hypothetical example, 14:00 UTC corresponds to 09:00 at UTC−5 and 15:00 at UTC+1. These are conversions for the stated offsets, not year-round rules for particular cities.

Time-zone boundaries and daylight saving rules are civil decisions, not facts dictated solely by longitude. NIST’s local-time FAQ explains how U.S. rules and exceptions work.

A location’s offset can change when daylight saving time begins or ends. That is why a fixed offset and a place name are not always interchangeable. For a future appointment, identify the date as well as the location, and check the offset that applies then.

What a leap second does

The first leap second in the current system was inserted on June 30, 1972. All leap seconds listed in NIST’s historical table have been positive: an extra second was added.

For a positive leap second, the UTC sequence is 23:59:59, then 23:59:60, then 00:00:00. That unusual extra clock reading is different from adding a day to the calendar or moving clocks for daylight saving time.

Leap seconds create discontinuities that can complicate digital systems. Different ways of handling them can also undermine consistent synchronization, a concern explicitly recognized by international measurement authorities.

What the 2035 decision actually says

In Resolution 4 of its 2022 meeting, the General Conference on Weights and Measures decided that the maximum permitted difference between UT1 and UTC would increase in or before 2035.

The resolution requested a proposed new limit that would allow UTC to remain continuous for at least a century, along with an implementation plan. This is more precise than saying that leap seconds were immediately abolished or that the resolution simply ordered the last one on a particular date.

The underlying trade-off is straightforward: allowing a larger difference from Earth-rotation time can avoid frequent one-second adjustments to the shared clock.

Practical habits

1. Include the full date and a clear time reference when arranging something across regions.
2. Check the relevant local offset for that date rather than assuming it never changes.
3. For recurring meetings, check what happens when daylight saving time begins or ends.

UTC supplies the common reference. Local rules determine the clock reading people use. Keeping those two layers separate prevents much of the confusion.