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Unix Timestamp Converter

Converts a Unix timestamp (seconds or milliseconds since 1970-01-01T00:00:00Z, auto-detected) to a UTC and local date-time, or an ISO 8601 date-time (with optional time, offset or IANA zone) to epoch seconds and milliseconds, plus weekday, day of year and ISO week.

When to use

You need to decode an epoch value from logs or an API, or produce the epoch seconds for a given calendar date and time.

Do not use when: You need Julian Day numbers (use julian-date) or wall-clock conversion between two cities (use time-zone-converter).

Formula

unix_seconds = floor((instant − 1970-01-01T00:00:00Z) / 1 s); |timestamp| > 1e11 → milliseconds, else seconds; datetime without offset: instant = wall time − offset(time_zone)

Unix time counts SI seconds ignoring leap seconds (POSIX definition), so every day has exactly 86,400 s. Supported instants: years 1 to 9999.

Inputs

Parameter	Type	Required	Description
`timestamp`	number	no	Seconds since the Unix epoch; values with \|timestamp\| > 1e11 are read as milliseconds. Takes precedence over datetime when both are given. Range: ≥ -1000000000000000, ≤ 1000000000000000
`datetime`	string	no	ISO 8601 date-time to convert instead of a timestamp: YYYY-MM-DD[THH:MM[:SS]] with optional Z or ±HH:MM offset; without an offset it is read in time_zone.
`time_zone`	string	default UTC	IANA zone used to interpret a datetime without offset and to report the local date-time.

Outputs

Output	Type	Unit	Description
`unix_seconds`	integer	s	Whole seconds since 1970-01-01T00:00:00Z (floored).
`unix_milliseconds`	integer	ms	Milliseconds since the Unix epoch.
`iso_utc`	string		Instant in UTC, e.g. 2023-11-14T22:13:20Z (milliseconds shown only when non-zero).
`utc_string`	string		RFC 7231 / RFC 1123 form, e.g. Tue, 14 Nov 2023 22:13:20 GMT.
`local_datetime`	string		Instant in time_zone with its UTC offset (ISO 8601).
`utc_offset`	string		Offset of time_zone at that instant (±HH:MM).
`day_of_week`	string		Day of the week of the UTC date.
`day_of_year`	integer		Ordinal day of the UTC date, 1 January = 1.
`iso_week_date`	string		UTC date in ISO 8601 week notation YYYY-Www-D.
`interpretation`	string		How the input was read (seconds, milliseconds, explicit offset or zone).

Example

Timestamp 1700000000: {"timestamp":1700000000} → {"unix_seconds":1700000000,"unix_milliseconds":1700000000000,"iso_utc":"2023-11-14T22:13:20Z","utc_string":"Tue, 14 Nov 2023 22:13:20 GMT","local_datetime":"2023-11-14T22:13:20+00:00","day_of_week":"Tuesday","day_of_year":318,"iso_week_date":"2023-W46-2"}

2026-09-23 09:00 in Australia/Adelaide: {"datetime":"2026-09-23T09:00","time_zone":"Australia/Adelaide"} → {"unix_seconds":1790119800,"iso_utc":"2026-09-22T23:30:00Z","local_datetime":"2026-09-23T09:00:00+09:30","utc_offset":"+09:30","day_of_week":"Tuesday","day_of_year":265}

GET https://tttkmbb.com/api/v1/calculate/unix-timestamp?timestamp=1700000000

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/unix-timestamp (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/unix-timestamp · Markdown: https://tttkmbb.com/time/unix-timestamp.md · JSON definition: https://tttkmbb.com/time/unix-timestamp.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="unix-timestamp"
OpenAPI operationId: convert_unix_timestamp
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

IEEE Std 1003.1 (POSIX) – Seconds Since the Epoch (standard)
Unix time (Wikipedia) (reference)
ISO 8601 – Date and time format (Wikipedia) (reference)

FAQ

How do you tell seconds from milliseconds?

Any absolute value above 1e11 is treated as milliseconds (1e11 s would be the year 5138), so 1700000000 is seconds and 1700000000000 is milliseconds.

Are leap seconds counted?

No. Unix time is defined without leap seconds; the 27 leap seconds inserted since 1972 are absorbed by repeating a second, so conversions here match POSIX and every standard library.

Related calculators

Julian Date Calculator — Astronomical Julian Day for the same instant.
Time Zone Converter — Wall-clock time of the instant in another zone.
Date Difference Calculator — Days between two calendar dates.