Date Calculator
Add or subtract years, months, weeks and days from any date to find a deadline, anniversary or future appointment.
How it works
Pick a start date (today by default), choose Add or Subtract, and enter any mix of years, months, weeks and days.
Years and months are applied first. If the resulting day doesn't exist, like February 31, it moves to the last day of that month.
Weeks and days are then added as exact 24-hour days. You'll see the weekday, day of the year, ISO week number and a calendar.
Formula
result
Where:
- Y, M
- = Years and months (calendar units)
- W, D
- = Weeks and days (fixed-length units)
Worked example
What's 90 days after October 2, 2026?
October has 29 days left, November 30, so 59 days brings you to November 30. The remaining 31 days land on December 31, 2026, a Thursday, day 365.
Projecting Deadlines and Milestones
This tool is essential for project management and professional scheduling, where you need to determine a finish date based on a fixed lead time. For example, if a contract requires a 90-day notice period, you can find the exact calendar date required to meet that legal obligation efficiently without manually counting weeks on a calendar.
When planning, remember that 'one month' is a variable unit of time. Adding a month to January 30th lands on February 28th, while adding exactly 30 days would land you in early March. Always clarify whether your deadline is based on calendar months or a fixed number of days to remain compliant with your agreements.
Date vs. Date-to-Date Calculator
Choose this tool when you have a start date and a duration (like '45 days from now') and need to find a destination. The Date-to-Date Calculator is designed for when you already have two specific dates in mind and need to measure the gap or chronological distance between them for reporting purposes.
Think of it like a map: this tool tells you where you will land if you travel a certain distance, while the other tells you how far apart two existing landmarks are. Use this for forward-looking projections and the other for retrospective tenure analysis or calculating the length of past events.
Common Pitfalls in Date Math
A frequent mistake is the 'fencepost error,' where users are unsure whether to count the start date as day one. By default, adding one day to Monday results in Tuesday, the start day is treated as zero. For project site days or inclusive event counting, you may need to manually add one to your result.
Note that this tool handles raw calendar math and does not automatically skip weekends or public holidays. If your deadline must fall on a business day, check the resulting weekday provided in the results and adjust your input days to land on a Friday or Monday to avoid scheduling conflicts.
Frequently asked questions
February 28, or February 29 in a leap year. When the target month is shorter, the date moves to that month's last day.