Date Duration Calculator
Count the days, weeks, months and years between two dates, or add and subtract time from a date to find a new one — all in your browser.
Frequently Asked Questions
Does it count the end day?
By default the end day is not counted, so the duration is the number of nights between the two dates. Tick the "Include the end day" checkbox to add one day and count both the start and end dates.
Can I add days to a date?
Yes. Switch to the "Add or subtract" tab, pick a start date, choose add or subtract, and enter any combination of days, weeks, months and years. The tool shows the resulting date and its weekday.
Does it handle leap years?
Yes. All math uses real calendar dates in your browser, so leap years, varying month lengths and February 29 are handled automatically and accurately.
Understanding the Date Duration Calculator
The Date Duration Calculator answers two of the most common date questions in one place. Its first mode counts the time between any two dates, showing the result as total days, as weeks plus leftover days, as whole months plus days, and as a full years, months and days breakdown. An optional checkbox includes the end day when you want both endpoints counted. The second mode adds or subtracts any mix of days, weeks, months and years from a start date and returns the new date along with its weekday. Everything runs entirely in your browser using real calendar dates, so leap years and uneven month lengths are always handled correctly.
How it works
Pick a mode with the tabs. In "Duration between dates," choose a start and end date; the tool reads each date at local midnight, takes the difference in milliseconds for the total day count, then derives weeks, months and the years/months/days breakdown using true calendar boundaries. If the end date falls before the start date, it shows the absolute duration and tells you so. Ticking "Include the end day" adds one day. In "Add or subtract," enter a start date, choose add or subtract, and type any amounts; the tool shifts the year, then month, then day-and-week offsets to land on the resulting date and its weekday.
Worked example
Suppose you start a project on March 1, 2025 and finish on June 21, 2025. The calculator reports 112 total days, which is 16 weeks and 0 days, or 3 months and 20 days, or 0 years, 3 months and 20 days. Ticking "Include the end day" bumps the total to 113 days. Switching to the second mode, if you add 90 days to March 1, 2025, you land on Friday, May 30, 2025 — useful for tracking a 90-day notice period, warranty window, or payment deadline.
Tips & common mistakes
- Leave "Include the end day" unchecked to count nights (e.g., a hotel stay); tick it to count both calendar days inclusively.
- If your end date is earlier than your start date, the tool automatically shows the absolute duration instead of a negative number.
- Use the years/months/days breakdown for ages and anniversaries, and the total-days figure for deadlines or billing periods.
- In add/subtract mode you can combine units at once — for example add 1 year, 2 months and 10 days in a single calculation.
- Adding months keeps real calendar lengths: one month after January 31 may land in early March, since February is shorter.
- Use Copy, CSV or Print/PDF in the results bar to save or share any calculation.