Local time versus UTC

Coordinated Universal Time, UTC, is the civil timescale used as a common reference. The Unix epoch is defined as 1970-01-01 00:00:00 UTC. When two systems log the same timestamp, they are talking about the same instant, even if one operator is in Chicago and another is in London.

Local time is that instant written with a time zone offset and, often, a daylight-saving rule. 15:00 in one city can be 20:00 UTC the same day, or 14:00 the previous afternoon, depending on the offset. The number in a Unix timestamp does not change when you switch the label. Only the printed clock changes.

The timestamp converter therefore asks you to choose a label. UTC prints the epoch’s native clock. Local time uses this device’s zone, the same zone the operating system already applies to the menu-bar clock. The page does not look up a city from a network address, and it does not download a zone database beyond what the browser already has.

ISO 8601 on the result panel ends with Z when the instant is shown in UTC. Z means Zulu, another name for UTC. If you copy only the digits and drop the Z, the next person may assume local time and shift the event by several hours. Keep the label with the value.

Daylight saving makes local clocks jump. UTC does not jump for that reason. If a log looks “an hour off” every March or November, compare the local line with the UTC line before you edit the stored timestamp. In most systems the stored value should stay in UTC (or as an epoch count) and the display should carry the zone.

When you press Now, both views describe the same current instant. If they disagree with a wall clock in the room, the device clock is the first place to look. This site does not correct that clock for you.