Complete guide to time tools

By the inyourbrowser.com editorial team · Reviewed and tested September 30, 2026

What are time tools?

Time tools convert between human-readable dates and machine-friendly representations, compute differences between two points in time, and translate moments across timezones. The category here covers Unix timestamp conversion, date arithmetic, and timezone conversion using the IANA database.

How to choose a time tool

Start from the value you have in hand and the answer you need back.

• If a log line, an API field, or a JWT expiry claim holds a bare number such as 1700000000, convert the epoch value to UTC and local time. Ten digits usually means seconds and thirteen usually means milliseconds, so pick the matching mode; the converter warns when the number looks like the other unit.
• If you have the date and need the number, for a database seed or a test fixture, run the same converter the other way: its Date to Timestamp mode reads your entry as local time and returns the value in both seconds and milliseconds.
• If the answer is a count of days between two calendar dates, such as the length of a billing period, measure the span between the two dates. The result is the difference between them, so 1 to 31 January gives 30 days; add one if both the first and the last day count.
• If you know a start date and a length instead, such as a 90-day notice period or a 14-day return window, add the days in the same calculator to get the end date, or enter a negative number to count back from a deadline.
• If the date is still ahead and you want to know how long is left, the calculator's Days until mode counts from today in days and weeks. For an exact age in years, months, and days, switch to its Age mode.
• If a meeting or launch time is set in one city and people elsewhere need their local time, look up the same moment in their zone for the actual date. The gap between two cities can change during the year when only one of them observes daylight saving time, or when they change clocks on different dates, and the converter states the difference for the moment you pick.
• If you hold a timestamp and need it in a zone other than your own, convert it to UTC first, then enter that UTC time in the timezone converter with UTC as the source zone.

For a side-by-side comparison with hosted meeting planners and the operating system clock, see the time tools overview.

Tool comparison

How to convert times and dates in the browser

1. Decide what time you need. Convert a timestamp, compute the gap between two dates, or move a time from one zone to another.
2. Enter the input. Paste a timestamp, pick two calendar dates, or select an origin time and timezone pair.
3. Choose units or zones. Switch between seconds and milliseconds, pick the source and target timezone, or enter a negative number of days to count back from a date.
4. Read and copy the result. Each tool has a Copy button for its result. The Unix timestamp converter also shows each moment as ISO 8601 (UTC), ready to paste into code or logs.

Common workflows

Schedule a multi-region standup. Pick the canonical meeting time, convert it into each participant's zone, then build the calendar invite with the offsets visible to everyone.

Audit log timestamps. Turn each epoch into a readable date, then count the calendar days between events when an incident spans several days of a postmortem timeline.

Track a long-running project. Count the days since launch, set up a countdown to the next milestone, and confirm the duration of each sprint matches the plan.

Time standards primer

Unix epoch time counts seconds since 1 January 1970 UTC. It ignores leap seconds, which keeps arithmetic simple at the cost of a few seconds of drift over decades.

ISO 8601 is the international standard for human-readable date-and-time strings. The format 2026-05-25T14:30:00Z is unambiguous, sortable as plain text, and parseable by every modern programming language.

The IANA timezone databaseis the source of truth for which offset to apply for a given moment in a given region. It covers historical rules going back decades, so the timezone converter can give correct answers for past dates as well as future ones.

Several common formats interoperate well with browser tools. RFC 3339 is a profile of ISO 8601 used by JSON APIs. Epoch seconds and milliseconds appear in log files, JWT claims, and many database adapters. UTC offset notation (+02:00, -05:00) communicates the local time and the fixed offset without naming a specific region, which is useful when the rule set itself is not material to the conversation.

Time arithmetic stumbles on three predictable edges: leap years (where February gains a day), leap seconds (occasionally inserted at the end of June or December to keep UTC aligned with astronomical time), and the daylight saving transitions that compress or stretch a single calendar day to 23 or 25 hours twice a year in most temperate regions. The tools here handle leap years and DST transitions correctly. Leap seconds are ignored to keep parity with the rest of the software ecosystem.

Frequently asked questions

What is a Unix timestamp?

The number of seconds (or milliseconds in JavaScript usage) that have elapsed since 1 January 1970 UTC. It is the canonical machine-friendly time format used by most APIs, databases, and log systems.

Why are there both seconds and milliseconds variants?

POSIX uses seconds. JavaScript Date objects use milliseconds. Many APIs adopt one or the other. The unix timestamp tool has a seconds mode and a milliseconds mode, and warns when a number looks like the other unit.

How are leap years handled?

The date calculator follows the Gregorian calendar: years divisible by 4 are leap years, except century years not divisible by 400. February 29 exists in 2024 and 2028 but not 2100.

What is the IANA timezone database?

A community-maintained directory of every timezone and its historical offset changes. Identifiers look like America/New_York and Europe/London. The timezone converter uses your browser's bundled copy of the database for conversions.

Does the timezone converter handle daylight saving?

Yes. The time you enter is read with the source zone's offset on that date, and the result uses the target zone's offset at the same moment, both from the IANA rules in your browser. If a clock change skips or repeats the time in the source zone, the tool shows a notice and says which offset it used.

What happens for dates before 1970?

Unix timestamps go negative. Most software handles them correctly, but some older systems clamp the value at zero. JavaScript Date arithmetic works for dates well before 1970, with the same Gregorian extension assumption.

Is the result affected by the user's clock?

Only where a tool compares with the present moment: the days-until countdown, the age calculator, the Now mode, and the relative lines such as “3 days ago” in the Unix timestamp and timezone converters. The converted dates themselves come only from the values you enter.

Can daylight saving shift a date calculation?

Adding 24-hour periods across a daylight saving change can land an hour off in local time. The date calculator avoids this: it works with dates at local noon, adds whole calendar days, and shows only the resulting date.

Related concepts

• Unix timestamp explains the count of seconds since 1970-01-01 UTC that the timestamp converter reads and writes.
• Daylight saving time covers the seasonal offset rules baked into IANA.
• JWT expiry and issued-at claims hold Unix timestamps in seconds, which the timestamp converter turns into dates.
• Calculator tools guide handles the broader category of arithmetic helpers.
• Developer tools guide connects to JWT decoding and cron scheduling.

Reviewed and tested September 30, 2026.