Date & Time Calculators
Age, date differences and countdowns.
Three calculators for exact age, the gap between two dates, and the countdown to an event. All of them handle leap years and uneven month lengths properly, which is where mental arithmetic and spreadsheet subtraction both tend to go wrong.
All 3 date & time calculators
Date arithmetic is harder than it looks
Dates resist the assumptions people bring to them. Months are four different lengths, years are two, the leap rule has an exception and the exception has an exception, and weeks do not divide into months at all. That is why date questions are worth a calculator even though each one feels like something you should be able to do in your head.
The leap year rule in full: a year is a leap year if it divides by 4, except centuries, unless they divide by 400. So 1900 was not a leap year and 2000 was. Any span crossing 29 February is a day longer than the same span elsewhere in the calendar.
Inclusive or exclusive: the difference that looks like a bug
The most common confusion in date arithmetic is not a mistake at all, it is two correct answers to two different questions.
| Question | Convention | Answer |
|---|---|---|
| How many days apart are they? | Exclusive, the gap | 11 days |
| How many days am I away for? | Inclusive, both ends | 12 days |
| How many nights is that? | The gap again | 11 nights |
Neither is wrong. A hotel counts nights, a contract usually counts the gap, and a holiday allowance counts both ends because you are absent on the first day and the last. The date difference calculator shows both figures side by side for exactly this reason, and states which is which so the pair cannot read as a contradiction.
Age is not a subtraction
Your age is not this year minus your birth year. It depends on whether your birthday has happened yet, so for roughly half the calendar that subtraction is a year too high. The age calculator works in years, months and days, and it labels the date it was computed for, because an age is only true on the day it is calculated. A page that says "36 years old" is wrong the morning after that person's next birthday, which is a real hazard on a static site and why the date is printed alongside.
Working days are a local question
Counting business days between two dates needs more than the calendar: it needs your jurisdiction's public holidays, which differ by country and often by region, and it needs to know whether your working week is Monday to Friday. These calculators deliberately count calendar days and say so, rather than quietly applying one country's holiday list to everybody.
Date & Time Calculators: common questions
How many days are between two dates?
It depends which question you mean. The gap between 1 and 12 March is 11 days; counting both end dates gives 12. The date difference calculator shows both and labels which is which, since neither convention is wrong.
Why does my age calculation seem a year out?
Because age is not the current year minus the birth year. It depends on whether the birthday has already occurred this year, so that subtraction is a year too high for roughly half the calendar.
Do these calculators handle leap years?
Yes. A year is a leap year if it divides by 4, except centuries, unless they divide by 400, so 1900 was not and 2000 was. Any span crossing 29 February is one day longer than the equivalent span elsewhere.
Can I count business days instead of calendar days?
These tools count calendar days deliberately. Business days depend on your country's and region's public holidays and on your working week, and applying one country's holiday list to everyone would produce confidently wrong answers.