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Time & date

Timestamps and dates, converted without a timezone surprise.

Written for: Developers and designers · Everyone else, quickly

What this family is for

A Unix timestamp is unambiguous to a computer and meaningless to a person, and a date string means different instants depending on the timezone it is read in. These tools convert between epoch seconds or milliseconds and ISO 8601, and say plainly which zone the result is in, so a value copied between a database, a log and a form stays the same instant.

Timestamps, date arithmetic and time zones

These tools split along a line that causes most date bugs: converting an instant versus computing a calendar interval. A Unix timestamp names a single instant and is timezone-free. A date difference counts calendar days, which depend on the timezone you are in. Mixing the two — subtracting two timestamps and calling the result “days” — gives a number that is wrong near a daylight-saving boundary.

The second split is between a date and a time. A bare date such as 2026-01-01 has no instant attached; reading it as UTC midnight makes it the previous day in a negative-offset timezone. Any tool here that takes a bare date documents how it resolves that.

If you have… Use Because
Turn an integer into a readable time Timestamp converter Seconds or milliseconds, formatted in UTC
The number of days between two dates Date difference Calendar days, with weekends optionally excluded
The week number of a date Week number ISO 8601 week numbering, with the year-boundary rule applied
A date N days from now Date difference (add/subtract) Calendar arithmetic, not seconds

4 tools in Time & date

Unix Timestamp Converter Convert between Unix timestamps in seconds or milliseconds and ISO 8601, with the UTC result and a relative time. Date Difference Calculator Count the days, weekdays, weeks and years between two dates, and add or subtract days from any date — with the calendar breakdown spelled out. Week Number Calculator Find the ISO 8601 week number and week-numbering year for any date, and see the Monday-to-Sunday range that week covers. Age Calculator Age from a date of birth on any date: years, months and days, total days, weeks and months lived, and the countdown to the next birthday.

Common mistakes with these tools

  • Treating milliseconds as seconds — A 13-digit number is milliseconds. Passing it to a seconds-based converter puts the date about 50,000 years in the future.
  • Subtracting timestamps to get “days” — That gives 24-hour periods, which is not the same as calendar days across a clock change. Use the date tool for calendar arithmetic.
  • Reading a bare date as a local date — Without a time and an offset, the instant is ambiguous. This site resolves bare dates as UTC and says so on the page.

Frequently asked questions

Is a Unix timestamp in seconds or milliseconds?

Both exist. JavaScript and many APIs use milliseconds; Unix and most server logs use seconds. This tool lets you choose, and the difference is exactly a factor of 1000 — 1,700,000,000 seconds is 1,700,000,000,000 milliseconds.

What timezone is the output in?

The ISO 8601 result is UTC, marked with a trailing Z. That is the unambiguous form; your browser can render it in local time for display.

Why does my date parse to the wrong day?

A bare date like 2026-01-01 is read as UTC midnight, which can fall on the previous day in a negative-offset timezone. Include a time and an offset when that matters.

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