This date calculator handles the two questions people ask about the calendar most often: how far apart are two dates, and what date lands a given number of days, weeks, months, or years before or after a starting point? Switch modes with a single click and get an instant answer.
Difference mode reports the gap as total days, weeks, calendar months, and a full years-months-days breakdown, and even splits the span into weekdays versus weekend days. Add/subtract mode is ideal for deadlines, contract terms, notice periods, warranties, and return windows — it returns the exact resulting date along with its day of the week.
How the Date Difference Is Calculated
The number of days between two dates is their calendar distance: October 10 minus October 1 is 9 days, because 9 midnights pass between them. Following the standard convention, the start date counts and the end date does not — count both ends only when you need an inclusive tally (add 1).
Weeks are simply days divided by 7 with the remainder shown separately. Months and years are trickier because calendar months vary from 28 to 31 days, so the calculator counts complete calendar months from the start date, borrowing the previous month's real length when the day-of-month math goes negative. That is why January 15 to March 10 is 1 month and 23 days, not "1.8 months".
Adding Months and Years: The Month-End Rule
Adding days or weeks is unambiguous — every day is 24 hours, every week is 7 days. Adding months and years follows the same-day convention with clamping:
- January 15 + 1 month = February 15 (same day of month)
- January 31 + 1 month = February 28, or February 29 in a leap year (clamped to the month's last day)
- February 29, 2024 + 1 year = February 28, 2025
- March 31 − 1 month = February 28 (subtraction clamps the same way)
This matches how banks compute monthly due dates and how most legal deadlines are interpreted. If a rule specifies "30 days" rather than "1 month", use the days unit — over February, the two can differ by 2–3 days.
Example: March 15, 2026 to July 4, 2026
Take the span from March 15, 2026 to July 4, 2026. Counting elapsed days: 16 remaining in March, plus 30 in April, 31 in May, 30 in June, and 4 in July gives 111 days.
That converts to 15 weeks and 6 days. In calendar terms it is 3 complete months (March 15 → June 15) plus 19 more days. Of the 111 days, 80 are weekdays and 31 fall on weekends — useful if you are estimating working time before a deadline.
In the other direction: what is 90 days after March 15, 2026? Sixteen days finish March, then April (30) and May (31) consume 77, leaving 13 days into June — June 13, 2026, a Saturday.
Frequently Asked Questions
How do I calculate the number of days between two dates?
Count the calendar days from the start date up to, but not including, the end date. For short spans, add the days remaining in each month along the way. For anything longer, a date difference calculator is far more reliable — leap years and 30- versus 31-day months make manual counting error-prone.
Does the count include both the start and end dates?
By convention, no — the result counts elapsed days, so Monday to Friday of the same week is 4 days. If your situation needs an inclusive count (for example, counting nights booked versus days present on a trip), add 1 to the result.
What is 30 days after January 31?
March 2 in a common year (28 days finish February, leaving 2 days into March) and March 1 in a leap year. Note this differs from "1 month after January 31", which is February 28 or 29 — days and months are not interchangeable near month ends.
How are months counted when the dates fall on different days of the month?
The calculator counts complete calendar months first: a month completes when the same day of the month recurs. Leftover days are then counted using the actual length of the month being crossed. So April 20 to June 5 is 1 month (April 20 → May 20) plus 16 days.
Why would I count weekdays separately from weekend days?
Many real deadlines only consume working time: shipping estimates, payroll processing, court filing windows, and project schedules typically run on business days. The weekday count here excludes Saturdays and Sundays but not public holidays, so subtract any holidays that fall inside your span.