Date Calculator
Calculate your Add or subtract from a date in seconds with the free Date Calculator. No registration needed.
Start date & offset
Resulting date
What is a Date Calculator?
A Date Calculator lets you add or subtract days, weeks, months, and years from any starting date to find a future or past date. It handles all the tricky calendar logic automatically — varying month lengths, leap years, and end-of-month rollovers — so you don't have to count days on a calendar yourself. People use it for project planning, setting deadlines, calculating contract end dates, planning vacations, or figuring out warranty expiration dates. Instead of manually flipping through a calendar and counting days, you enter a start date and an offset, and the calculator returns the exact resulting date along with the day of the week and the total number of days between the two dates.
How to calculate dates manually (the formula)
To add or subtract time from a date manually, you work with the calendar's structure. Start with your start date. For days, simply count forward or backward on a calendar. For weeks, multiply the number of weeks by 7 and add or subtract those days. For months, advance or go back the given number of months on the calendar, being careful with months that have different lengths (January has 31 days, February has 28 or 29, etc.). For years, add or subtract the number of years — but remember that leap years add an extra day to February. The key formula is: Result Date = Start Date ± (Days + Weeks × 7) ± Months ± Years. The challenge is that months and years don't have a fixed number of days, so most people use an automated tool to avoid errors.
Example calculation
Suppose today is March 15, 2025 and you want to find the date 90 days from now. Enter March 15, 2025 as the start date, set the operation to Add, and enter 90 in the Days field. The calculator adds 90 days to March 15: March has 31 days, so March 15 + 16 days = March 31. That leaves 74 days. April has 30 days, so April 1 + 30 = April 30. That leaves 44 days. May has 31 days, so May 1 + 31 = May 31. That leaves 13 days, bringing you to June 13. The result is June 13, 2025. The calculator also tells you it is a Friday and that 90 days separate the two dates.
Common mistakes
- Forgetting about leap years — Adding one year to February 29 of a leap year should give February 28 of the next year (or February 29 if the next year is also a leap year).
- Ignoring month lengths — Adding one month to January 31 should give February 28 (or 29 in a leap year), not March 3. Different months have 28, 29, 30, or 31 days.
- Off-by-one day errors — When counting days between dates, forgetting to include or exclude the end date can change the result by one day.
- Confusing weeks with business days — Adding 2 weeks (14 calendar days) is not the same as adding 10 business days, which excludes weekends.
- Mixing AM/PM with 24-hour time — When combining date and time calculations, using the wrong time format can shift the result by 12 hours.
Frequently asked questions
How do I add or subtract days from a date?
Enter your start date, choose Add or Subtract, then enter the number of days, weeks, months, or years. The calculator instantly shows the resulting date, accounting for month lengths and leap years automatically.
How do I calculate the date 90 days from today?
Set the operation to Add, enter 90 in the Days field, and leave the start date as today. The calculator will show you the exact date 90 days in the future.
How do I subtract months from a date?
Select Subtract, enter the number of months you want to go back, and the calculator handles end-of-month adjustments automatically so you always get a valid date.
Does this calculator account for leap years?
Yes, the calculator automatically accounts for leap years and varying month lengths across all months, ensuring every result is accurate.
Can I calculate a date in the past?
Yes, simply select Subtract and enter the amount of time you want to go back. The calculator works for both future and past dates.
How accurate is the result?
Results are exact to the day. The calendar logic handles all date calculations precisely, including leap years and months with different lengths.