Date arithmetic is deceptively awkward: months have different lengths, leap years break the pattern every four years except when they do not, and "within 30 days" and "within 30 business days" can be six weeks apart. These tools do the counting so an off-by-one does not become a missed deadline.
Which one do you want?
| If you are… | Use |
|---|---|
| Days between two dates, or a deadline | Date Calculator — with a business-days mode that skips weekends. |
| Exact age, or a countdown to a birthday | Age Calculator — in years, months and days. |
| Unix timestamps in logs or code | Timestamp Converter — epoch seconds to a readable date and back. |
A distinction worth getting right on anything contractual: calendar days count every day including weekends and holidays, while business days skip them. A 30-business-day term is roughly 42 calendar days.
Where date arithmetic disagrees with itself
- Inclusive and exclusive counting differ by a day. Monday to Friday is 4 days between and 5 days inclusive. Rental periods, medication courses and ticket validity usually count inclusively; most legal deadlines exclude the start day and include the end day.
- Adding a month is not reversible. 31 January plus one month clamps to 28 February, and 28 February minus one month is 28 January — you do not land back where you started. For anything scheduled, count days or anchor every date to a single starting point.
- Business days are not four-fifths of calendar days. A 30-business-day term is about 42 calendar days, and public holidays stretch it further. Contracts that say days without qualifying the word are the ones that cause arguments.
- The leap-year rule has an exception people forget. Divisible by 4 is a leap year — except century years, which must also divide by 400. 2000 was; 1900 was not; 2100 will not be.
Frequently asked questions
Do these handle leap years?
Yes. A year is a leap year if divisible by 4, except century years, which must be divisible by 400 — so 2000 was a leap year and 1900 was not.
Does the business-day count skip public holidays?
It skips weekends only. Public holidays differ by country and by state, so subtract the ones that apply to you.
Why do two age calculators disagree by a day?
Time zones. Age is computed from your device's local date, so a birthday near midnight can land either side.