Date Calculator
Add or subtract days from a date. Start date plus or minus N days = result date.
Free · Runs in your browser · No signup
Examples to try
Date math
Result date = start date ± N days on the calendar (including month/year rollovers).
Common mistakes
- Counting business days when the tool counts calendar days.
- Timezone confusion around midnight deadlines.
- Off-by-one when both start and end dates are inclusive.
Scope: Calendar day arithmetic. For differences between two dates, use date difference.
How to Use
Enter your values in the fields above and click Calculate to get instant results. All computations run locally in your browser. No data is ever uploaded or stored.
Related Calculators
Date Addition & Subtraction
CalcSolver converts your start date to a timestamp, adds or subtracts the exact number of days, then converts back to a calendar date. Month boundaries and leap years are handled automatically.
Result date = start date + (N days × 86,400,000 ms)
Adding 90 days to January 15 gives April 14 (not 15 — February has 28/29 days). End-of-month dates roll naturally.
Common Day Offsets
7 days: one week · 14 days: one fortnight · 30 days: one month (approx)
60 days: two months · 90 days: one quarter · 180 days:** half a year · 365 days: one year
For business-day calculations, exclude weekends and public holidays — our calculator counts calendar days for clarity.
How Date Arithmetic Works
Date arithmetic — adding or subtracting a number of days from a given date — is more complex than simple addition because months have different lengths and leap years insert an extra day. CalcSolver's date calculator handles all of these variations automatically. When you enter a start date and a number of days, the calculator advances day by day, crossing month and year boundaries as needed, and returns the exact resulting date.
The Core Formula
The simplest form of date arithmetic is: Result Date = Start Date + N days, where N can be positive (future) or negative (past). Internally, the calculator converts the start date to a serial day count, adds N, then converts back to a calendar date. For month-based arithmetic, adding 1 month to January 31 yields February 28 (or 29 in a leap year) — the calculator adjusts to the last valid day of the target month. Adding 3 months to August 31 gives November 30, not November 31, because November has only 30 days.
Real-World Applications
Pregnancy due dates: Doctors calculate a due date by adding 280 days (40 weeks) to the first day of the last menstrual period. If the last period started on September 1, 2026, the estimated due date is June 8, 2027.
Legal deadlines: Many legal filings have deadlines expressed as "X days from the event." A 30-day appeal period starting from a judgment date of October 15 expires on November 14. Filing one day late can forfeit your rights, making precise calculation essential.
Project management: Sprint planning in agile development often uses 2-week (14-day) iterations. Starting a sprint on a Monday, adding 14 days lands on a Monday two weeks later. Adding 90 days to a project kickoff date gives the end-of-quarter checkpoint. Use CalcSolver's date difference calculator to verify the span between two known dates, and the work hours calculator for business-day-based planning.
Calendar Systems and Edge Cases
The Gregorian calendar, used worldwide, has 365 days per year with a leap day every 4 years (with century exceptions). Other calendars — Hebrew (lunisolar), Islamic (lunar, ~354 days/year), and Chinese (lunisolar) — have different month lengths and leap rules. This calculator uses the Gregorian calendar exclusively.
Edge cases to be aware of: adding 1 month to January 29, 30, or 31 in a non-leap year gives February 28. Subtracting 1 month from March 31 gives February 28 (or 29). These end-of-month adjustments ensure you always get a valid calendar date.
Tips for Accurate Date Calculations
- For "X business days from today," remember that weekends and holidays are not subtracted automatically — you need to account for them manually or add extra days.
- When a contract says "within 30 days," clarify whether it means calendar days or business days — the difference can be over a week.
- To subtract days, enter a negative number in the days field. Subtracting 90 days from today gives the date three months ago.
- Use ISO format (YYYY-MM-DD) dates in documentation to avoid international ambiguity.
- Combine with CalcSolver's date difference calculator to cross-verify your results.
Sponsored Content
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Frequently Asked Questions
How do I add days to a date?
Enter the start date and the number of days to add. The calculator counts forward, accounting for varying month lengths and leap years, and shows the resulting date.
How do I subtract days from a date?
Enter the start date and the number of days to subtract. Use a negative number or select the subtract option. The calculator counts backward from the start date.
Can I add weeks or months instead of days?
Yes, you can add or subtract days, weeks, or months. Adding 3 months to January 31 gives April 30 (adjusted for the shorter month). Adding 4 weeks is the same as adding 28 days.
What happens when I add a month to January 31?
Since February has only 28 days (or 29 in a leap year), adding 1 month to January 31 gives February 28 or 29 — the calculator adjusts to the last valid day of the target month. This is standard behavior across most date libraries and spreadsheet applications.
How do I calculate a due date 90 days from now?
Enter today's date as the start date and type 90 in the days field. The result is the exact date 90 days (about 3 months) from today. For pregnancy due dates, use 280 days (40 weeks) from the last menstrual period start date.
Can I subtract days to find a past date?
Yes. Enter the start date and a negative number of days (e.g., -30) to go backward in time. Subtracting 365 days from today gives approximately the same date one year ago.