Where our data comes from, and how we count
Every figure on this site comes from somewhere, and every count is made on a stated basis. This page says which, for all of it.
How we count days
Ask how many days remain until a date and you can get several different answers, all of them arrived at honestly. Two things decide the number, and a tool that does not tell you which it picked is asking you to trust a figure you cannot check.
The time zone decides which day it is
A day is not a global thing. At any given moment it is already tomorrow in Auckland while it is still today in Los Angeles, so the same question has two correct answers at the same instant. We count in a time zone you choose, we name that zone under every answer, and we default to the one your browser reports.
The counting basis decides whether today is one of them
From Tuesday to Friday is three days if you do not count the day you are standing on, and four days if you do. Neither is wrong. Notice periods and payroll usually count inclusively; everyday questions usually do not. We default to exclusive, we say so under every answer, and you can change it on any page.
Those two choices together are the entire reason two date calculators can disagree. Stating them costs nothing, so we state them.
The time zone database
Time zone rules come from the IANA time zone database, which is the reference implementation the whole industry uses. We do not ship a copy of it. Every conversion reads the copy already inside your browser, which means a rule change reaches you when your browser updates rather than when we get round to redeploying, and it means you are not downloading a second copy of something you already have.
There is a consequence worth being plain about: we cannot tell you one version number and have it be true for everybody. Your browser's copy may well be newer than the one our build machine carries, and if it is, yours is the one that applies to what you see. For the record, this build was made on against tzdata 2024b with ICU 76.1.
We do not claim an accuracy figure for the clock itself. It reads your device's clock, so it is exactly as right as your device is, and we have no way to measure that from inside a web page. Quoting a precision we cannot demonstrate would be worse than saying nothing.
Sources and licences
Everything below is used under a licence that permits it, and the licence is named so you can check.
| Source | Used for | Licence |
|---|---|---|
| IANA time zone database | Every time zone conversion, every offset, every daylight saving rule | Public domain |
| date-holidays | Public holiday dates by country | ISC AND CC-BY-3.0 |
| Nager.Date | Cross-checking public holiday dates | MIT |
| astronomy-engine | Sunrise, sunset, moon phase and solar position | MIT |
| Archivo, by Omnibus-Type | All prose and interface text | SIL OFL 1.1 |
| Martian Mono, by Evil Martians | Every figure on the site | SIL OFL 1.1 |
| Tabler Icons | The marks beside tool names | MIT |
Attribution
Public holiday data is derived in part from date-holidays, used under the Creative Commons Attribution 3.0 licence. Time zone data is from the IANA time zone database, which is in the public domain. Archivo and Martian Mono are used under the SIL Open Font Licence 1.1 and are served from this domain rather than a third party.
Corrections
This is a site about being right, so being told we are wrong is useful rather than unwelcome. If a figure here disagrees with one you trust, the first thing to check is the time zone and the counting basis printed under it, because that explains most disagreements. If it is not that, it is a bug and we would like to know.
Questions
Why do two date calculators give different answers?
Almost always for one of two reasons. The first is the time zone: it is already tomorrow in Auckland while it is still today in Los Angeles, so at a single instant the honest answer to "how many days until" differs by one depending on where you are standing. The second is the counting basis: some tools count today as day one and some do not. Both choices are legitimate. The problem is that most tools make one silently.
Which basis does DateTools use by default?
Exclusive counting, in your own time zone as your browser reports it. Exclusive means the day you are on now is not counted, so from Tuesday to Friday is three days. You can switch to inclusive counting on any page, and the choice is remembered.
How accurate is the clock?
The clock reads your own device's clock. It is exactly as accurate as your device is, and we have no way to measure that from inside your browser, so we do not claim a figure. What we can promise is that the conversion between zones is right, because it uses the IANA rules your browser already carries.
Do you update when daylight saving rules change?
The rules come from your browser, not from us, so a change reaches you when your browser updates rather than when we redeploy. That is deliberate. Bundling our own copy of the time zone database would mean shipping a second, staler copy of something you already have.
What happens with dates before the Gregorian calendar?
Dates before the Gregorian reform are handled as proleptic Gregorian, which means the modern rules are projected backwards. That is the standard computing convention and it is what your browser does. It is not what a historian means by a date in, say, 1500, and where a page depends on that distinction the page says so.