About UnixClock

UnixClock is a set of free tools for working with time, dates and time zones. Every calculation happens in your browser: nothing you type is uploaded and no account is needed. Whether page views are counted is a setting of each deployment, and the privacy policy — generated from that same setting — says exactly what this one loads.

Why it exists

Time is one of those subjects that looks simple and is not. Offsets are political rather than geographic, daylight saving rules differ between countries that share a longitude, abbreviations collide, and months are not a fixed length. Most mistakes in this area come from applying an intuition that is nearly right — "New York is five hours behind London", "a month is thirty days" — to a case where it is not.

The tools here are built to give the correct answer and to say why, particularly in the cases where the obvious answer is wrong. Where something is genuinely ambiguous, they say so rather than guessing.

How it works

Time zone conversions use the IANA time zone database that ships with your browser, so daylight saving is applied by rule rather than from a stored offset, and a government's rule change reaches the site as soon as your browser updates. Astronomical figures use the NOAA solar equations and published lunar algorithms. Date arithmetic is anchored to UTC midnight so that a clock change in your own zone can never shift a result by a day.

The site is static. There is no back end, no database and no server-side processing, which is what makes it fast and what makes the privacy claims easy to keep: there is nowhere for your data to go.

What it is not

Every clock here reads from your own device, so the site is exactly as accurate as your computer. It is not synchronised with an atomic clock or a time server, and it should not be treated as an authoritative time source. The clock check tool measures how far your device has drifted.

Holiday lists are a planning convenience rather than a legal record. Astronomical times are calculated for an idealised horizon and will differ from what you observe. Translations other than English may contain errors. Where being wrong is expensive — payroll, legal deadlines, aviation, navigation — verify against an authoritative source.

Corrections

If a result looks wrong, it is worth checking two things first: whether your device clock is accurate, and whether your browser is recent enough to know about a rule change. Beyond that, corrections are welcome and the contact page explains what to include so a fix can be made quickly.

How the tools are tested

Correctness in this area is easy to claim and hard to demonstrate, so the calculations are covered by an automated suite that runs on every build. It checks known-good values rather than merely checking that the code runs: that the March 2024 equinox falls at 03:06 UTC, that London to New York is about 5,570 kilometres, that the sun is overhead at 23.44° at the solstices, and that a holiday falling on a Saturday produces the observed Monday its country actually uses.

Several of those tests exist because something was wrong first. A daylight saving edge case resolved a skipped hour backwards into the gap it was meant to escape. A clock accuracy check reported a false margin because it assumed its own polling interval was honoured. Each fix came with a test so the same class of error cannot return quietly.

Accuracy, stated plainly

Time zone conversions are as accurate as the IANA database in your browser, which is the same source operating systems use. Sunrise and sunset are within about a minute for most inhabited latitudes, degrading near the poles. Distances are within roughly half a percent, since the Earth is treated as a sphere. Lunar phase times are within a couple of minutes. Holiday dates are generated from published calendar rules and are a planning aid rather than a legal record.

Who it is for

Mostly people coordinating across zones — remote teams, anyone with family abroad, developers reading timestamps in logs — and people who need a date calculation they can trust rather than one they have done in their head. The pages are written to be useful to both: the answer first, then enough explanation to see why it is the answer.

Design principles

Three rules shape the tools. Answer first: the result appears without a submit step wherever the inputs make that possible. Say what is uncertain: where a calculation rests on an assumption — a flat horizon, an average sleep cycle, a clamping rule for month arithmetic — the page states it rather than presenting an approximation as a fact. And never guess silently: an ambiguous date or a skipped daylight saving hour produces a question, not a confident wrong answer.

The site is also built to be fast on a slow connection. Pages are static, no tool waits on a request to anyone else, and the whole interface is a single stylesheet and a few kilobytes of JavaScript per tool.