FIELD NOTE 03 / TIME
Unix timestamps, UTC and local time
A timestamp and a clock reading are not interchangeable. One names an instant on a shared timeline; the other is a local display of that instant under a timezone rule.
Check whether the value is seconds or milliseconds
Unix-style timestamps are commonly represented as seconds from the Unix epoch, 1970-01-01 00:00:00 UTC. JavaScript’s Date time value is measured in milliseconds from that epoch. The Measureline tool asks which unit you have rather than guessing from the number, because old dates and synthetic values can make magnitude-based guessing unreliable.
For example, the same instant can be represented as 1,000 times as many milliseconds as seconds. If you select the wrong unit, the result shifts far from the intended date.
UTC is the portable reference
UTC gives a common baseline. The tool shows an ISO-style UTC value ending in Z, then renders the same instant in the timezone configured on your device. The local display may differ by several hours and may use a different date. It is a display conversion, not a second instant.
Local wall time is incomplete without a zone
“2026-11-08 at 1:30 AM” does not identify one instant on its own. The timezone and daylight-saving rules matter; in some places a clock time can occur twice or not at all around a clock change. A datetime-local field contains no timezone by itself. This tool interprets that value using the browser’s local timezone, so its reverse conversion may differ on another device.
Data and precision limits
JavaScript Date accepts a bounded range of millisecond values and represents time at millisecond granularity. The timestamp tool rejects non-finite values and dates outside the browser’s supported range. It discards fractional milliseconds and does not parse named zones or leap-second notation. For distributed systems, store an unambiguous timestamp or a local date-time together with an explicit IANA timezone.
Try the tool
Open the timestamp converter. Compare the UTC output with the local display and verify the seconds/milliseconds selection against the source system.