Date Calculator — Add, Subtract and Compare Dates
Add or subtract time from any date, then compare two dates by total days, weeks, weekdays, and exact calendar distance.
Why date math is easy to get wrong
Adding 30 days is not the same thing as adding one calendar month. February, leap years, and month-end boundaries make naive arithmetic fail quickly. This calculator keeps those two ideas separate so you can choose the correct interpretation for contracts, deadlines, subscriptions, invoicing, or planning.
It also helps when you need more than one representation of a date difference. In project work, you might care about total days. In HR or legal paperwork, you may need completed years, months, and days. In operations, you may care about weekdays between two dates. This page gives all three views in one place.
Frequently asked questions
How do I add 90 days to a date?
Enter the start date in the add-or-subtract panel, type 90 in the Days field, and leave the other amounts at zero. The result appears immediately with its weekday. Ninety days is not the same as three months: from January 31, 2026, 90 days later is May 1, while three calendar months later is April 30.
How do I count the days between two dates?
Use the comparison panel and enter both dates. The total-day figure counts the start date as day zero, so January 1 to January 2 is one day. If you need the number of calendar dates including both ends, for example for a hotel stay counted in nights versus days, add one to the result.
Why does adding one month to January 31 give February 28?
A month is a calendar unit, not a fixed number of days. February 31 does not exist, so the calculator uses the last valid day of the target month. This month-end rule matches how most contracts, billing systems, and spreadsheet functions such as EDATE handle the same case.
Does the calculator account for weekends and holidays?
The comparison panel reports weekdays (Monday to Friday) as well as calendar days, but it does not remove holidays. For deadlines stated in business days, use the working days calculator, which can also exclude observed US federal holidays.
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Get the date you actually mean
How to add or subtract a date
Enter the starting date, then enter a positive number to move forward or a negative number to move backward. Years and months are applied first, with the day clamped to the end of the target month if needed. Weeks and days are applied afterward. A week always means seven calendar dates in this tool.
The comparison panel shows completed years, months, and remaining days alongside a total-day count. Those are alternative descriptions of the same interval, not quantities to add together. The weekday count excludes the start date and includes the end date if it is Monday through Friday. It does not subtract holidays.
A month is a calendar unit
A calendar month can be 28, 29, 30, or 31 days. From January 31, 2026, adding one month gives February 28. Adding 30 days gives March 2. If you add both one month and one day, the month-end rule is applied first, giving March 1. Choose the unit used by your schedule rather than replacing every month with 30 days.
Set the start to January 31, 2026. Set Months to −1 and the other amounts to 0. The result is December 31, 2025. Then set Months to 1 to see February 28, 2026.
A common mistake to avoid
The interval from January 1 to January 2 is one day. If you need the number of calendar dates including both endpoints, add one to the total-day result.