"How many days?" has at least three answers

Ask how many days there are from 1 January to 3 January and you can defend both 2 and 3. Ask how many days you have until a deadline a month away, and the honest answer is a lot fewer than 30. Date arithmetic is full of these small disagreements, so Omikuji Tools has three calculators that each take one of the questions seriously and state which rule they use.

Date Calculator: say which rule you mean

The first half gives the difference between two dates: days, weeks-and-days, years/months/days, hours and minutes, plus how many of those days are weekdays (Mon–Fri) and how many fall on a weekend. The checkbox that settles the argument above is Include the first day. Leave it off and 1–3 January is 2 days; tick it and it is 3, which is how hospital stays and campaign periods are usually counted.

The second half goes the other way: a base date plus or minus N days, business days, weeks, months or years, with the weekday of the result. Two rules are stated on the page, and they are worth knowing before you use the answer. Business days skip only Saturdays and Sundays; public holidays are not considered. And adding months clamps to the end of the month, so one month after 31 January is 28 or 29 February. You can copy the results or share a URL that reproduces the calculation.

Age Calculator: the age on a particular day

Enter a date of birth and optionally an "as of" date, and you get the exact age in years, months and days, days and weeks lived, the weekday you were born on, the Chinese zodiac, the star sign and a countdown to the next birthday. The "as of" date is for forms that ask for someone's age on 1 April rather than today.

It also covers some things that are specific to Japan: Japanese era dates (Meiji to Reiwa, so birth dates from 1868 onwards), the traditional kazoe-doshi age (1 at birth, plus one every New Year's Day), milestone ages such as 60 (kanreki) and 88 (beiju), and the years of school entry and graduation under the Japanese April school year. You can save several people and see all their ages on one screen, and that list stays in your browser.

Workday Countdown: the days you can actually work

This one begins from the idea that "a whole month left" is an illusion. You set a deadline, and the big number is the workdays remaining once weekly days off, holidays and your own one-off days off are taken out. Today is not counted because it is already under way, and by default neither is the deadline day. For a single date, exception workdays override everything else, and the page states the order: exception workdays > extra days off > holidays > weekly days off.

Add tasks and it divides what is left by the workdays remaining, giving you days per task (or tasks per day). You can keep several projects, and export them as JSON to move them to another browser. Japanese public holidays can be switched on as days off; they are on by default only in the Japanese version. A share URL carries the deadline and the days-off settings but never your tasks.

👉 Date Calculator · Age Calculator · Workday Countdown — free, no sign-up, runs in your browser.

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