Date Calculator — Add or Subtract Time
Add or subtract years, months, weeks and days from any date, with correct handling of month lengths and leap years.
How to use this calculator
- 1Pick a start date and whether you are adding or subtracting.
- 2Fill in any combination of years, months, weeks and days — they are applied together.
- 3Leave fields at zero if you do not need them.
How the calculation works
Result = Start ± years ± months ± (weeks × 7 + days)- Years, months
- Calendar units — the day of the month is preserved where possible
- Weeks, days
- Fixed-length units of exactly 7 and 1 day
Calendar units are applied before fixed-length units, because the order changes the answer when month-end clamping is involved.
When the target month is too short for the start day, the result clamps to the last day of that month. 31 March minus one month is 28 February in a common year.
Leap years are handled automatically: 29 February plus one year gives 28 February.
Worked example
10 August 2026 plus 3 months and 10 days
- 1.Add 3 calendar months to 10 August 2026, giving 10 November 2026.
- 2.Then add 10 days, giving 20 November 2026.
- 3.That is 102 days after the start date.
Result: 20 November 2026 (a Friday)
Calendar arithmetic is not the same as day arithmetic
Adding "three months" to a date is not the same operation as adding ninety days to it, even though both feel like a rough quarter of a year. Three months from 1 April is 1 July — a gap of 91 days. Three months from 1 December is 1 March — a gap of 90 days in a common year, but 91 days whenever the intervening February is a leap year and picks up its extra day. Calendar units track a position on the calendar; fixed-length units track a raw span of time, and the two only agree by coincidence.
That distinction is why this calculator treats "years and months" as one kind of input and "weeks and days" as another, and applies them in that order — calendar units first, so that any month-end adjustment happens before the fixed-length days are added on top.
What happens when the target month is too short
Add one calendar month to 31 January and there is no 31 February to land on — the month simply does not have that many days. The standard convention, used here and by most spreadsheets and scheduling software, is to clamp the result to the last valid day of the target month: 31 January plus one month becomes 28 February, or 29 in a leap year, not 3 March.
This detail matters more than it looks. A billing date set for the 31st of the month will quietly shift to the 28th or 30th in shorter months under this convention, which is why some recurring payments and subscriptions appear to move around depending on the month — the arithmetic is consistent, but the calendar itself is not evenly spaced.
Where this kind of date arithmetic gets used
Projecting a date forward or backward from a known starting point is one of the most common small calculations in everyday planning.
- Deadlines and due dates — invoices, tax filings and legal responses are frequently defined as a fixed number of days or months from a triggering event.
- Warranty and return windows — coverage and eligibility to return an item typically run for a stated period from the purchase or delivery date.
- Notice periods — resignations, lease terminations and cancellations often need to land on or after a specific calculated date to be valid.
- Recurring billing and renewal dates — subscriptions and memberships project forward by a fixed interval, month after month, from the original sign-up date.
- Follow-up scheduling — medical check-ups, project milestones and reminders are commonly set as "N weeks from today" rather than a specific date chosen by hand.
Leap years, and why the calendar needs them at all
A year — one full orbit of the Earth around the Sun — takes about 365.2422 days, not a round 365. Ignoring that extra quarter-day would drift the calendar out of sync with the seasons by roughly one day every four years, and by a full month within a couple of centuries. The leap year, an extra day added periodically, exists purely to keep the calendar aligned with the solar year.
The Julian calendar, in use across much of the Western world for over 1,500 years, added a leap day every four years without exception, which overcorrected slightly and drifted about three days off the solar year every four centuries. By the sixteenth century that drift had pushed the calendar ten days out of alignment, prompting Pope Gregory XIII to reform it in 1582 with the refinement still in use today: a leap year every four years, except century years, unless they are also divisible by 400. That single exception — 1900 was not a leap year, but 2000 was — is what keeps the modern Gregorian calendar accurate to within about a day every 3,300 years.
What this assumes, and where it stops
Assumptions
- The Gregorian calendar applies.
- Dates carry no time component.
Limitations
- Does not skip weekends or public holidays. Use the business days calculator for working-day offsets.
- Month-end clamping follows the most common convention, but some legal and financial contexts use different rules for month-end dates.
Common questions
What is 31 January plus one month?
28 February, or 29 in a leap year. Since 31 February does not exist, the standard convention clamps to the last valid day of the target month. This calculator follows that convention, as do spreadsheets and most date libraries.
Why does adding "1 month" not always add the same number of days?
Because months are between 28 and 31 days long. Adding a month preserves the day of the month rather than a fixed day count — which is usually what people mean by "in a month".
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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