Date calculator with working days and your holidays

The calculations people need

Between two dates: Calendar days, weeks and days, months and days, and working days, all at once, because people often need more than one.
Add or subtract: A date plus 30 calendar days, or plus ten working days, or minus six weeks: the deadline calculator most offices need.
Age or duration: Years, months and days since a date, for anniversaries, service length or a project's age.
Day of the week: What day a date falls on, and which week number it is, with the week-numbering rule stated.
Countdown: Days left until a date, as a quick view; for a big display, the countdown timer app is the better tool.

Rules to decide and show

Most disagreements about date counts are really disagreements about rules. Pick one for each and put it on screen beside the result.

Inclusive or exclusive. From Monday to Friday is four days if you count the gap, five if you count both ends. Offer a switch, default to excluding the start date, and say which is on.
Working week. Monday to Friday by default, editable for businesses that work Saturdays or close on a different day.
Holidays. A list of dates that don't count as working days. Public holidays differ by country and region, and some move every year, so the list must be yours, and kept current.
Landing on a non-working day. When a deadline falls on a weekend or holiday, roll it forward to the next working day, back to the previous one, or leave it. Contract terms and court rules sometimes specify which.
Month arithmetic. Adding one month to 31 January has no perfect answer. Say whether you clamp to the last day of the month.

For anything with legal weight, such as a notice period, a statutory deadline or a court filing, check the rule that applies and use the calculator as a convenience rather than the authority.

Your holiday list

The app has no network, so it can't fetch an official holiday calendar. Instead, the holidays are written into it, or entered by the user. Two good patterns:

Built in. Give the list in your description: this year's and next year's public holidays for your region, plus company closure days. The model writes them into the app. Check every date, and plan to ask for an update each year.
Editable. Ask for a holidays screen where users add, remove or paste dates, saved in their browser. Useful for teams in several countries, each with its own list.

Describe it to Anticode

"A date calculator for a small accounting practice. Three tabs: between two dates (calendar days, weeks and days, working days), add or subtract days (calendar or working), and duration in years, months and days. Working week Monday to Friday, editable. Built-in public holidays for England and Wales for this year and next (dates below), plus an editable list of office closure days saved in this browser. Inclusive or exclusive switch, off by default. For deadlines landing on a non-working day, a choice of next working day, previous working day or no change. Show the rules applied beside every result. A list of the holidays counted in a result. Keyboard-friendly date inputs."

Then test it against cases you know: a period spanning a long holiday weekend, a deadline that lands on a Sunday, a leap-year February. Fix anything that disagrees in plain words.

Publish it at its own address and link to it from your site, intranet bookmarks or email signature; it can also be installed to a phone or desktop home screen. Published apps can't be embedded inside another website, so it opens as its own page. If you'd rather it sit within your own site, ask for it as one section of a larger app build.

Limits

No live holiday data. Lists are fixed in the app or typed by users, and must be kept up to date.
No time zones or hours unless you ask for them; for that, see the time zone converter.
No calendar integration. It can't read or add events to anyone's calendar.
Not legal advice on deadlines.

Related: the project timeline maker for planning with dates, and the accountants page for practice websites.

Questions people ask

Does the date calculator count the end date?

It does whatever you choose. Ask for an inclusive or exclusive switch and for the rule to be shown with each result.

Will it know next year's bank holidays?

Only if they're written in or added by users. It can't fetch them, so update the list each year.

Can I embed it on my existing website?

No. Published apps can't be framed by other sites. Link to it instead, or install it from the browser.