TK TaskKit
Developer Tools

Timestamp Converter

Convert between Unix seconds, milliseconds, and ISO 8601. Pick a timezone, see derived day-of-year and ISO week.

Timezone
Type a Unix timestamp or ISO 8601 date in any field, or click Now.

Inputs stay on this device. Every developer tool on TaskKit runs entirely in your browser. Tokens, payloads, and pasted text are not transmitted to TaskKit servers or third parties.

What this tool does

Converts between Unix timestamps (seconds and milliseconds), ISO 8601 strings, and human-readable date components. Picks up your browser timezone by default, lets you switch to any IANA zone, and shows derived fields you'd otherwise compute by hand: day of year, ISO week, weekday.

When you'd use it

  • Reading an exp claim out of a JWT and seeing it in your local time.
  • Filing a bug that mentions "Tuesday at 14:00 UTC" — quickly converting both directions.
  • Confirming whether a Unix value is in seconds or milliseconds (1700000000 vs 1700000000000).
  • Generating an ISO 8601 string for a config file or API call.

How it works

Date math uses the browser's Date and Intl.DateTimeFormat APIs, with a hand-built table of IANA timezone names. The "is this seconds or milliseconds?" detection looks at the magnitude — anything under 1e12 is treated as seconds, anything above as milliseconds, which works correctly until the year 33658.

Notes

Why does ISO 8601 have a Z at the end? Z means "UTC offset of zero" — short for "Zulu time." 2026-05-03T11:34:00Z and 2026-05-03T11:34:00+00:00 mean the same instant.

ISO week vs day-of-year? ISO weeks (1-53) are defined so week 1 contains the first Thursday of the year. Day-of-year (1-366) is just the ordinal day. They can disagree at year boundaries — Jan 1, 2024 was day 1 but ISO week 1 of 2024 actually started Jan 1.

Does this handle leap seconds? No, and neither does JavaScript. The browser's clock smears them. For applications that care, use a leap-second-aware library on the server.

Related tools