How the date duration calculator works
A duration is simply the distance between two points on the calendar. The calculator converts each date to a day count, subtracts to get the total number of days, and then expresses that span in friendlier units — weeks, months, and a years-months-days breakdown.
Total days = End date − Start date
Because it uses real calendar dates, varying month lengths and leap-year February 29s are counted correctly without any manual adjustment.
Counting inclusively vs. exclusively
There are two reasonable ways to count days. Exclusive counting treats the start date as day zero and gives the gap between the two dates — useful for "days until" a deadline. Inclusive counting adds one so both endpoints are included — useful when you're totaling the days of a trip or an event that runs through its last day. Toggle the checkbox to switch between them.
Adding and subtracting dates
The second mode answers the other common question: "what date is 90 days from now?" Pick a starting date, enter the number of days or weeks, choose add or subtract, and the calculator returns the resulting date along with the day of the week — handy for project deadlines, return windows, and waiting periods.
Everyday uses
- Deadlines: how many days until a project is due.
- Trips: the number of nights between check-in and check-out.
- Milestones: days since an anniversary or a start date.
- Waiting periods: the date a 30- or 90-day window closes.
Frequently asked questions
How do you calculate the number of days between two dates?
Convert both calendar dates to a common reference and subtract. This calculator counts the whole days from the start date to the end date using real calendar dates, so different month lengths and leap years are handled automatically.
Does the duration include both the start and end date?
By default it counts the gap between the dates — the start date is day zero. Tick "include end date" to count both endpoints, which is how you would total the number of days in a hotel stay or an event that runs through its final day.
How are years and months in the breakdown worked out?
The years-months-days breakdown counts whole years first, then whole months since that point, then the leftover days against the actual length of each calendar month — the same way you would naturally describe a span as "2 years, 3 months, and 10 days."
Can I add or subtract days from a date?
Yes. Switch to "Add or subtract" mode, pick a starting date, enter a number of days (or weeks), and choose add or subtract. The calculator returns the resulting calendar date and the day of the week it falls on.
Does this account for leap years?
Yes. All math uses real calendar dates, so February 29 in leap years is counted correctly in both the duration and the add/subtract modes.
Disclaimer: Durations are measured between calendar dates only and do not account for time of day or time zones.