Date Calculator
Add or subtract days, weeks, months, and years from any date to find the resulting date and day of the week.
Adds or subtracts a span from any date. Months and years adjust for varying month lengths and leap years.
How to use this calculator
Choose a start date, select Add or Subtract, and enter any combination of years, months, weeks, and days. You can fill in one field or all four — only the non-zero values affect the result. The output updates instantly, showing the result date and the day of the week it lands on. To measure the gap between two specific dates instead of adding a duration, use the days between dates calculator.
How date arithmetic works
Adding or subtracting calendar time is more nuanced than simple arithmetic because months have different lengths and leap years add an extra day to February. The calculator handles this by applying each unit in a defined order:
- Years first: The year field is shifted by the number of years entered. If the resulting year is a non-leap year and the start date is February 29, the date is adjusted to February 28.
- Months second: The month field is shifted next. If the resulting month is shorter than the start day — for example, adding 1 month to January 31 targets February 31, which doesn't exist — the date is clamped to the last valid day of the month (February 28 or 29).
- Weeks and days last: These are converted to a total number of days and added or subtracted from the running date as exact calendar increments.
This ordering means mixed inputs like "1 month and 15 days" are applied sequentially, not simultaneously. The result can differ slightly from what you might expect if you tried to add them in a different order.
Days vs. months — an important distinction
Many common timelines specify a period in days rather than months, and the two are not equivalent. Adding 30 days always moves exactly 30 calendar days forward. Adding 1 month moves to the same day number in the next calendar month, which could be 28, 29, 30, or 31 days depending on which month you land in.
For practical purposes: use days when a legal document, warranty, or contract specifies a day count. Use months when the agreement references calendar months (e.g., a 3-month subscription ending on the same date three months later).
Worked examples
Adding days: a 90-day return window
A retailer offers a 90-day return policy starting from the purchase date. If you bought an item on March 1, adding 90 days gives May 30 — the last day to return it. (March has 31 days, April has 30, so March 1 + 90 days = May 30.)
Adding months: a 6-month contract
A service contract starts on August 31 and runs for 6 months. Adding 6 calendar months lands on February 28 (or 29 in a leap year) because February has no 31st. The calculator clamps to the last valid day automatically.
Subtracting time: a notice period
You need to give 60 days' notice before a lease ending on December 1. Subtract 60 days from December 1 → notice must be given by October 2.
How to interpret your result
The result shows both the date and the day of the week. The day of the week matters for deadlines: if your calculated date falls on a Saturday or Sunday and the deadline requires a business day, you may need to act the preceding Friday or the following Monday, depending on the applicable rules. This calculator does not automatically skip weekends or public holidays — adjust manually if your situation requires business days only.
Common mistakes to avoid
- Mixing up days and months for the same calculation. If a contract says "30 calendar days," enter 30 days — not 1 month. They can differ by 1–3 days.
- Forgetting the end-of-month clamp. Adding months to a date near the end of the month can shorten the span unexpectedly. Adding 1 month to January 31 gives February 28, not March 2.
- Not accounting for leap years. Adding 1 year to February 28, 2024 gives February 28, 2025 — but adding 1 year to February 29, 2024 also gives February 28, 2025, since 2025 has no February 29.
- Ignoring weekends for business deadlines. If your result is a Saturday or Sunday, a business-day deadline typically moves to Friday (before) or Monday (after). Check the applicable rule.
- Confusing addition and subtraction. "30 days before the due date" means subtract 30 days from the due date, not add 30 days to a start date. Double-check the direction of your calculation.
How we calculate this
Date addition and subtraction apply each unit in sequence: years shift the year field, months shift the month field (clamping to the last valid day if the target month is shorter), weeks and days add or subtract exact calendar days. All calculations use real calendar arithmetic, correctly handling leap years and varying month lengths.
Frequently asked questions
How do I add days to a date?
Pick a start date, choose 'Add,' and enter the number of days (or weeks, months, or years). The calculator returns the resulting date and the day of the week it falls on. Days are added in exact calendar increments, so 30 days from January 15 is February 14.
How do I subtract time from a date?
Choose 'Subtract' and enter the span you want to go back. For example, subtracting 90 days from today gives you the date 90 calendar days ago, adjusted for the actual length of each month. This is useful for finding lookback periods, statute-of-limitation windows, or return deadlines.
How are months and years handled differently from days?
Days and weeks are exact counts of calendar days. Months and years shift the calendar date field by field — add one month to January 31 and you land on the last day of February (28 or 29 depending on leap year), because February has no 31st. Mixing units is applied in order: years first, then months, then weeks, then days.
What is the difference between adding 30 days and adding 1 month?
Adding 30 days always moves exactly 30 calendar days forward. Adding 1 month moves to the same day number in the next month, which could be 28, 29, 30, or 31 days depending on the month. For billing cycles and contract terms, clarify whether the agreement uses calendar months or a fixed day count.
What can I use this calculator for?
Common uses include: project deadlines ('what date is 45 business days from today?'), warranty and return windows, notice periods in contracts, visa or permit expiry dates, construction or renovation timelines, and any 'what date is X from now?' question.
Does the calculator handle leap years?
Yes. Because the calculator works with actual calendar dates, February 29 in leap years is recognised. Adding 1 year to February 29, 2024 lands on February 28, 2025 (since 2025 is not a leap year), which is the standard convention.
How do I find how many days are between two dates instead?
This calculator adds or subtracts a duration from a single date. To measure the gap between two known dates, use the days between dates calculator, which gives you the total days, weeks, and a years/months/days breakdown.