h2check

Convert a timestamp across timezones

Paste a Unix timestamp or an ISO 8601 string to see the same instant in several timezones at once, which is how a date that is a day out in one of them becomes visible. Runs in your browser.

What it checks

  • Which unit a bare number was read as, by digit count: seconds, milliseconds, microseconds or nanoseconds.
  • What the same number would mean in the other unit, since a 1970 date in a log is almost always a seconds value in a milliseconds field.
  • The instant as Unix seconds, Unix milliseconds, ISO 8601 in UTC, and the GMT form HTTP requires in Date and Last-Modified.
  • The wall-clock date and time in every zone you name, each with its offset.
  • Whether those zones fall on more than one calendar day, which is the bug that breaks a report grouped by day.
  • Which of the zones change offset during the year, and which are on summer time at that instant.
  • A value past 2147483647, which a signed 32-bit time_t cannot hold.
  • The difference between a date-only ISO string, which ECMAScript reads as UTC, and a date-and-time string with no offset, which it reads as local.

What is not stored

Nothing is stored and nothing is sent. This checker is JavaScript that runs inside your browser tab, so the value you paste never reaches h2check.org or any other server. Close the tab and it is gone.

Limits

  • The zone rules come from the timezone database inside your own browser. A server on an older ICU build can place a transition on a different day, which is one real cause of an hour-out bug that will not reproduce locally.
  • Historical dates are converted with the current rules for the zone. Offsets before about 1970 are approximations in every implementation.
  • Leap seconds do not exist in Unix time or in JavaScript Date, so a one-second difference against a system that counts them is expected rather than a defect.
  • Nothing in a bare number says whether it is seconds or milliseconds. The unit here is inferred from how many digits it has.

Procedures that explain the result

Put it on your own page

Link to it with the value filled in, and the checker runs as soon as the page opens.

<a href="https://h2check.org/tools/timestamp/">Convert a timestamp across timezones</a>