Date Calculator
Add or subtract days, weeks, months, or years from any date — or count the days between two dates. Results appear instantly, with leap years and month lengths handled automatically.
Enter a date
Enter an amount
Result date
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How it works
Days and weeks use simple day arithmetic: September 15, 2026 + 30 days = October 15, 2026. Months and years use calendar arithmetic: the month or year changes first, and if the target day does not exist (February 31, for example), the date clamps to the last day of that month. Differences count full years, then full months, then the remaining days.
Example
Start date September 15, 2026, operation Add, amount 30, unit Days → result date October 15, 2026. That is a total of 30 days and a calendar difference of 1 month.
Frequently asked questions
How do I add days to a date?
Keep the mode on Add or subtract, enter your start date, choose Add, type the number of days, and leave the unit on Days. The result updates instantly: September 15, 2026 plus 30 days is October 15, 2026. You can also add weeks (1 week equals 7 days), months, or years by changing the unit selector.
How do I subtract days from a date?
Pick Subtract in the Operation list and enter the number of days to remove. September 15, 2026 minus 30 days lands on August 16, 2026, because August has 31 days. The same option works for weeks, months, and years, which is useful for finding due dates, deadlines, or contract start dates.
What happens when adding a month lands on a shorter month?
Calendar arithmetic clamps to the last valid day of the target month. January 31, 2026 plus 1 month gives February 28, 2026, because February 2026 has only 28 days; in a leap year the result would be February 29. The same rule applies to years, so February 29, 2024 plus 1 year becomes February 28, 2025.
How do I count the days between two dates?
Switch the mode to Difference between two dates and enter both dates. The calculator shows the total number of days, the same span in weeks and days, and the calendar breakdown in years, months, and days. For example, September 15 to October 15, 2026 is 30 days, or 4 weeks and 2 days.
Does the day count include the start or end date?
The count excludes the start date and includes the end date, matching how deadlines and durations are usually measured: September 15 to October 15, 2026 equals 30 days. If you need an inclusive count that includes both endpoints, add 1 to the total days shown in the result.
How are leap years handled?
The calculator follows real calendar rules, so February 29 exists in leap years such as 2024 and 2028 and is skipped in other years. Crossing a leap-day period adds one extra day: March 1, 2023 to March 1, 2024 spans 366 days, while March 1, 2024 to March 1, 2025 spans 365 days.
Can I count weeks between two dates?
Yes. In difference mode every result includes a weeks-and-days line, so 30 days reads as 4 weeks and 2 days. In add or subtract mode you can also pick Weeks as the unit; adding 2 weeks moves the date exactly 14 days. Week arithmetic always uses 7-day weeks.
Is my data saved or sent anywhere?
No. All date math runs entirely in your browser with client-side JavaScript, so the dates you enter never leave your device. There are no accounts, no logs, and no uploads. Once the page has loaded, you can even disconnect from the internet and keep calculating.