Choose what you want to count
A date difference measures the gap between two calendar dates. Counting every date on a list is a different task: March 10 to March 13 has a gap of three days, but contains four dates when both ends are included. Decide which convention you need before using the result.
Elapsed days versus including both dates
3 elapsed days. There are three steps between the dates: 10 to 11, 11 to 12, and 12 to 13. This is the calculator’s default difference result.
This illustration changes the counting explanation only. The calculator above always reports elapsed calendar days.
In difference mode, entering the same date twice gives zero days. If your task counts that date as one included calendar date, add one to the difference. This arithmetic adjustment does not determine the counting rules of a contract, booking, or official deadline.
The calculator sorts the two dates before finding their difference. Reversing them gives the same positive gap, not a negative result. Use Add or subtract days when you want to move backward from a known date.
Worked examples you can enter above
A four-date event with a three-day gap
Choose Difference between two dates. Enter March 10, 2026 as Start date and March 13, 2026 as End date. The result is 3 days. A list that includes all event dates has four entries: March 10, 11, 12, and 13.
Thirty days after January 15
Choose Add or subtract days, enter January 15, 2026, select Add days, and enter 30. The result is February 14, 2026. January 15 is the starting point, not day one; January 16 is one day later.
Seven days before March 3
Choose Subtract days with March 3, 2026 and 7 days. The result is February 24, 2026. There is no need to enter a negative number: the Operation selector sets the direction.
Crossing a year boundary
Add 10 days to December 25, 2026 to reach January 4, 2027. The calculator rolls into the next month and year automatically; you do not need to split the calculation at December 31.
For an independent check, enter the starting date and calculated result in difference mode. Their gap should match the number of days added or subtracted. Whole weeks are the day count divided by seven with the remainder omitted: a 10-day gap shows one total week, with three days left over.
Month lengths and leap days matter
Days are not interchangeable with months. January 31, 2026 plus 30 days is March 2, 2026, while adding one calendar month under a last-day-of-month convention would reach February 28. The add/subtract mode takes days only, not months or years.
February 28 to March 1 is one day in 2025 and two days in 2024, because February 29 exists in 2024. In the Gregorian calendar, a year divisible by four is a leap year except for century years that are not divisible by 400. Thus 2000 was a leap year and 2100 will not be.
How this tool expresses the calendar span
The secondary years/months/days result counts complete calendar months from the earlier date, using the last valid day when the original day does not exist in the target month. It then counts the remaining days. Twelve complete months are shown as one year.
Under that convention, January 31 to February 28, 2026 is 0y 1m 0d and January 31 to March 1 is 0y 1m 1d. The elapsed totals are 28 and 29 days respectively. Calendar-span conventions can differ between systems; use the total days when you need an unambiguous calendar-day gap.
Calendar days are not business days
This tool counts every calendar day, including weekends and holidays. It does not load a country's holiday calendar, apply a work schedule, or move a result to the next working day.
For example, Friday March 6, 2026 plus three calendar days is Monday March 9. If you instead count three Monday-to-Friday workdays after that Friday, ignoring holidays, you reach Wednesday March 11. The two answers use different rules.
For an official deadline, establish the applicable start-day, holiday, weekend, and cutoff-time rules separately. This calculator supplies date arithmetic, not those rules. Do not interpret its result as an automatic business-day deadline.
Why a result may differ by one day
- Different endpoints: one person counts the starting date and another counts the gaps between dates.
- Different units: “one month” is treated as 30 days even though the calendar month has a different length.
- A leap day: an interval crosses February 29 in a leap year.
- A date was interpreted differently: verify the month and day in your browser's date picker, especially when sharing dates between regions.
- A time-of-day assumption: this tool takes dates only, so it cannot calculate precise elapsed hours or an exact timestamp deadline.
The day-gap calculation compares calendar dates independently of daylight-saving clock changes. A day on the calendar is counted once even if the local clocks make it shorter or longer than 24 hours. This is not a timezone converter.