Convert between Unix Timestamps and dates/times in the browser
Convert second/millisecond Timestamps to UTC or a specified TimeZone, and also generate Unix seconds/milliseconds from dates and times. Input is not sent externally.
Convert UNIX time and dates/timesConclusion: Specify the input date-time time zone, convert it to an instant in UTC, then obtain the timestamp
Enter a wall-clock time in the date and time field, then select UTC or Asia/Tokyo. Check the resulting UTC, offset, Unix seconds, and Unix milliseconds.
Do not casually interpret date-time strings without an Offset as Local. First confirm the TimeZone and units expected by the API or DB.
Do not determine the unit from the digit count alone; check the value's source, unit, and expected date/time range together.
Uniquely determine an instant from a wall-clock time.
Apply IANA time zone rules to a calendar date and clock time to obtain a UTC instant. Nonexistent times at the start of DST are not converted, and repeated times at the end can produce multiple candidates.
For ambiguous times, the current tool adopts the earlier candidate and shows a warning. It has no UI for choosing a candidate.
| Account to check | What to check | judgment |
|---|---|---|
| Date | Year/month/day, hour/minute/second, ms | Whether it exists |
| TimeZone | IANA name | Input region or |
| DST | Missing and duplicates | Whether there are warnings |
| output | Seconds / milliseconds | Whether it is a destination specification |
Procedure for checking with the Unix Timestamp Converter
- Enter a date and time.
- Select the input TimeZone.
- Check UTC and Offset.
- Copy the required Unix seconds or milliseconds.
A date and time alone cannot uniquely determine a timestamp. Always specify a timezone.
The flow from a date and time to a timestamp
Apply timezone rules to the entered wall-clock time to obtain a UTC instant, then represent the epoch difference in seconds or milliseconds.
- Validate calendar date / time
- Apply the IANA time zone
- Convert to UTC Instant
- Output epoch difference
Distinguish seconds and milliseconds from JavaScript Date units.
A typical Unix timestamp is the elapsed number of seconds since 1970-01-01 00:00:00 UTC. In contrast, JavaScript Date, Date.now(), and getTime() handle milliseconds since the same epoch. When passing seconds to Date, multiply by 1000; when obtaining Unix seconds from Date milliseconds, divide by 1000 and convert to an integer as appropriate for the use case.
| example | Candidate unit | How to check |
|---|---|---|
| 1723456789 | Seconds | Check whether the conversion result is in the expected era and whether the API specification uses seconds. |
| 1723456789000 | Milliseconds | Check the units of JavaScript Date and API specifications |
| 1723456789000000 | Microseconds candidate | Do not enter it directly into the current tool; divide by 1000 according to the specification |
| 1723456789000000000 | Nanoseconds candidate | Avoid Number precision issues and use processing that supports String and BigInt. |
Separate the Timestamp instant from the TimeZone display
A Unix Timestamp is a number representing one instant; it does not embed a display TimeZone such as "JST Timestamp" or "UTC Timestamp." Displaying the same value in UTC, Asia/Tokyo, and America/New_York changes the Calendar Date and Clock Time.
Adding a fixed nine hours can explain Asia/Tokyo's current offset, but it cannot be used for general time-zone conversion. Let the IANA time zone and Intl.DateTimeFormat determine the offset and DST rules applicable at that instant. When converting back from a date-time without an offset, first determine which time zone's wall clock it represents.
Distinguish the trailing Z in UTC notation, Offsets such as +09:00, and IANA TimeZone names. The same string becomes a different Timestamp if the TimeZone interpretation differs.
How to isolate issues when the date and time are wrong
For a one-hour offset, check DST; for a nine-hour offset, check the UTC and JST interpretation.
Treat values such as 2026-02-30 as invalid and correct them instead of relying on automatic date rollover.
- Whether the input format is datetime-local
- Did you select a TimeZone?
- Check whether it is at a DST boundary
- Check whether seconds and ms are confused.
Supported scope of the current Unix Timestamp Converter
DevelopTools' Unix Timestamp Converter converts Unix seconds or milliseconds to date and time, and calculates Unix seconds and milliseconds from a date and time with an IANA time zone. Processing is completed in the browser and does not send or save input values or conversion results to a server.
| Item | Current tool operation |
|---|---|
| Input Unit | Automatic detection, seconds, milliseconds. Automatic detection estimates that integer parts with 12 or more digits are milliseconds |
| Date and time display | UTC, selected TimeZone, ISO 8601, RFC 3339, date, time, weekday, difference from now, UTC Offset |
| TimeZone | Device, UTC, choices such as Asia/Tokyo, and any IANA time zone |
| Reverse conversion | Generate Unix seconds and milliseconds from a datetime-local wall clock and time zone |
| Special dates and times | Supports negative timestamps, dates before 1970, and dates after 2038 within the JavaScript Date range. |
| Daylight saving time | Treat nonexistent wall times as errors, and select the earlier candidate for duplicate wall times with a warning |
| Unsupported units | Direct input and automatic conversion of Unix Microseconds / Nanoseconds are not supported |
Automatic detection by digit count is only a convenience estimate. It can misidentify past or future values, leading zeros, custom epochs, microseconds, and nanoseconds, so always compare it with the source system specification and expected date range.
Check against primary sources for the specification and runtime
Verify JavaScript Date epoch milliseconds, parse, toISOString, and Intl.DateTimeFormat against MDN and the ECMAScript specification. Check RFC 3339 for internet date/time representations, RFC 7519 for JWT NumericDate, and the integer representations and official documentation of the runtime, OS, and database for the 2038 problem.
Example: convert 2026-08-12 12:30 JST to Unix seconds
Enter a date and time and Asia/Tokyo.
In UTC, it is 03:30 on the same day, and the seconds and milliseconds corresponding to that Instant are displayed.
- Enter date and time
- Select Asia/Tokyo
- Check UTC
- Copy Unix seconds
Asia/Tokyo currently does not observe DST, but offsets vary seasonally in other regions.
Frequently asked questions
- Is UNIX time in seconds or milliseconds?
- A typical Unix Timestamp is in seconds, but JavaScript Date and some APIs use milliseconds. Treat 10 and 13 digits only as a guide; check the source specification and expected date range.
- Is UNIX time UTC?
- A Timestamp represents one instant as elapsed time since the Epoch. UTC and JST are applied when displaying that instant as a human-readable date and time.