Date Add / Subtract Calculator
Pick a start date, then enter how many years, months, weeks and days to add or subtract. You instantly see the resulting date, its weekday, and the total number of days between start and result — handy for contract deadlines, milestone planning, due-date arithmetic and "days from today" questions.
Please enter a valid number
Result date
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Total day offset
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Formula
Result = Start ± (years × 12 months) ± months ± (weeks × 7 days) ± days Months/years are applied first; if the target month is shorter than the source day, the day clamps to the last valid date of that month.
- · Uses the proleptic Gregorian calendar. Months/years are added first, then weeks and days — reverse order could differ by a day across leap years.
- · Day clamping: e.g. Jan 31 + 1 month → Feb 29 (leap) or Feb 28 (common). Useful for "same-day-of-month" deadlines that fall on a 31-day month.
- · Feb 29 + 1 year clamps to Feb 28 in a non-leap year and stays on Feb 29 when the target year is a leap year.
- · Weekday is derived from the absolute UTC day count, so daylight-saving time and time zones never shift it.
- · All arithmetic happens locally in your browser — no data leaves your device. The link is shareable because inputs are encoded in the URL query string.
- · Supports years AD 1–9999. Inputs that overflow this range return an error rather than a result.
Frequently asked
If I add one month to January 31, what date do I get?
The tool clamps to the last day of the target month — Feb 29 in a leap year, otherwise Feb 28. This matches the EDATE convention used by Excel / Google Sheets and most financial contracts, which is convenient for monthly recurring deadlines that fall on a 31st. If you wanted "31 days later" instead, switch to adding 31 days (you would land on March 2 or 3 depending on the year).
Does the order — months first vs. days first — change the result?
Yes — when leap years or differing month lengths are involved, "months first" and "days first" can disagree by a day. This tool follows the same order as SQL, Excel, Java, and most accounting software: apply years and months first, then weeks and days. Picking one consistent order avoids ambiguity.
Can I just type negative numbers in the input fields?
No — every numeric field accepts non-negative integers only. To go backwards in time, flip the "Add / Subtract" toggle to "Subtract (past)" and every field is applied as a subtraction. This avoids the ambiguity of mixing minus signs across multiple fields.
Why is the date range capped at years 1–9999?
ISO 8601 and the HTML <input type="date"> control both use a four-digit year, so 9999 is the natural upper bound; BC dates use a different convention outside this tool. Any practical scheduling — contracts, visas, retirement planning, project deadlines — fits comfortably inside this range.
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