WatchTimeDate Duration Calculator
Find the exact gap between two dates — in days, and broken down into years, months, and days — or jump forward and backward from a date by a number of days.
Days between two dates
Add or subtract days
Both calculators run entirely in your browser. Counting convention: the difference excludes the start date and includes the end date, so Monday to the following Monday is 7 days.
How the difference is calculated
Counting days between dates sounds trivial and isn't, because our calendar is irregular. Months are 28, 29, 30, or 31 days long, and years are 365 or 366. The reliable method — the one this tool uses — is to convert each date to a plain day count (days since a fixed reference point) and subtract. That sidesteps month lengths entirely and cannot be confused by leap years.
The years-months-days breakdown is a separate calculation with a genuine judgment call in it: from January 31 to March 1 is "1 month and 1 day" only if you decide that adding a month to January 31 lands on February 28 (or 29). This tool follows the convention used by most legal and payroll systems: advance whole years first, then whole months (clamping to the end of a short month), then count the remaining days.
Leap years in one paragraph
A year is a leap year if it's divisible by 4 — except century years, which must be divisible by 400. So 2024 and 2028 are leap years; 1900 was not, but 2000 was. The reason is that a solar year is about 365.2422 days: adding a day every 4 years slightly overcorrects, and skipping three century years out of four brings the average calendar year to 365.2425 days, close enough that the calendar drifts by only about one day every 3,200 years.
Inclusive or exclusive counting?
"How many days between Friday and Monday?" has two defensible answers: 3 (exclusive of the start, the convention used here and by most software) or 4 (counting both endpoints, common in travel bookings and prescription durations). Before you rely on a count for anything contractual — notice periods, visa windows, medication schedules — check which convention the other party uses. Off-by-one errors on deadlines are the classic failure mode of date math.
Handy day-count references
| Period | Days |
|---|---|
| 1 week | 7 |
| 30-day month vs 31-day month | 30 / 31 |
| 1 common year | 365 |
| 1 leap year | 366 |
| Average month (365.2425 ÷ 12) | ≈ 30.44 |
| 90 days from Jan 1 (common year) | April 1 |
| 180 days from Jan 1 (common year) | June 30 |
Frequently asked questions
Does the calculator count weekends?
Yes — it counts calendar days, including weekends and holidays. Business-day counting varies by country and by company holiday schedule, so a raw business-day number without a holiday calendar attached is usually misleading.
Why does "1 month later" from January 31 give February 28?
Because February has no 31st. The standard convention clamps to the last day of the target month, which is what banks, payroll systems, and this calculator do.
Can I calculate across a leap day?
Yes. Because the tool subtracts absolute day counts, February 29 is simply one more day in the total whenever the range crosses it.
Calculating someone's age specifically? The age calculator handles birthdays, including February 29 births.