Unix timestamp and UTC date calculator

Convert Unix timestamps and work with calendar dates using explicit UTC rules. Choose an operation before entering a value.

How to use this tool

Choose timestamp conversion, calendar-day addition or date difference. The source label changes to show the required format.
For UTC-to-Unix conversion use YYYY-MM-DDTHH:mm:ssZ, optionally with one to three fractional-second digits. For calendar operations use YYYY-MM-DD.
Select Calculate date or timestamp, then review, copy or download the plain-text result. Load example demonstrates a negative epoch.

Scope and limits

All calculations use the proleptic Gregorian calendar and UTC. Years 0001 through 9999 are supported. Invalid dates, automatic date rollover, leap seconds, timezone offsets and local daylight-saving rules are rejected or unsupported.

Unix input must be a signed integer in the selected seconds or milliseconds unit; decimal fractions and exponent notation are rejected. UTC-to-seconds conversion rejects nonzero milliseconds instead of rounding.

Calendar-day addition preserves UTC midnight. Date difference is second date minus first date in whole days; it is not an elapsed-hours calculation across a local timezone. Leap-year rules are applied, including century exceptions.

Inputs start empty. Load example fills them only when you choose it. Reset clears every text field and output. Cancel stops the worker and keeps your input.

Processing runs locally. This tool does not upload or autosave your input. The site can load consent-controlled third-party services; use nonsensitive test input and keep passwords, tokens and personal records out of these fields.

Worked example

Input and settings

Operation: Unix timestamp to UTC
Unix timestamp: -1
Unit: seconds

Result

UTC timestamp: 1969-12-31T23:59:59.000Z
Unix seconds (exact decimal): -1
Unix milliseconds: -1000

The signed integer -1 is one second before the Unix epoch. The same instant is -1000 Unix milliseconds.

Unix timestamp and UTC date calculator

Input

Output

Worked example

Input and settings

Result

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