Date Calculator (Add or Subtract Time)
Add or subtract a span of years, months, weeks, and days from any date. The calculator returns the resulting calendar date along with its day of the week, applying the standard month-end clamping rule so the result is always a real date.
Calculate
Default result: Feb 28, 2026
Result
Result date
- Day of week
- Saturday
How to calculate
Pick a start date, choose whether to add or subtract, then enter any combination of years, months, weeks, and days. Leave fields at zero when you do not need them. The result updates as you type.
result = baseDate ± duration, applied in order years → months → weeks → days. Month and year steps use calendar arithmetic and clamp to the last valid day of the target month (Jan 31 + 1 month → Feb 28/29). Weeks count as 7 days. Subtracting negates every duration field before the same arithmetic runs.
Example calculation
Adding 1 month to January 31, 2026 clamps to the last valid day of February, giving February 28, 2026 — a Saturday.
- resultDay
- February 28, 2026
- dayOfWeekName
- Saturday
Assumptions
- Durations are applied in order: years → months → weeks → days.
- Month-end clamping is used: if the target month is shorter than the start day, the result clamps to the last day of that month (Jan 31 + 1 month → Feb 28 in 2026, or Feb 29 in a leap year). Likewise Feb 29 + 1 year → Feb 28.
- Weeks are treated as exactly 7 days.
- Subtract simply negates the durations and runs the same calendar arithmetic.
- When you open a shared link the inputs come from the URL first, then from your last-used values saved in this browser, then from the calculator's defaults.
Common mistakes
- Expecting the day-of-month to overflow into the next month — Jan 31 + 1 month is Feb 28 (clamped), not Mar 3. The calculator clamps to the last valid day instead.
- Assuming months and years are a fixed number of days. They are calendar-based, so adding 1 month can move the date by 28, 29, 30, or 31 days depending on the month.
Frequently asked questions
What happens when I add a month to January 31?
The result clamps to the last day of the target month. January 31 plus 1 month is February 28 in a normal year, or February 29 in a leap year. The date is never allowed to spill into the following month.
In what order are the durations applied?
Years first, then months, then weeks, then days. Years and months use calendar arithmetic with month-end clamping; weeks and days are then added as plain 24-hour days.
How does subtracting work?
Choose Subtract and the calculator negates every duration field, then applies the same calendar arithmetic. Subtracting 1 month from March 1 gives February 1.
Is the day of the week affected by daylight saving time?
No. The calculator counts whole calendar days, so the resulting date and its day of the week never shift because of a clock change.