Date Calculator — Add or Subtract Days

Adding and subtracting time from a date

Date arithmetic sounds trivial until months get involved. Days and weeks are fixed-length, so "45 days from now" is unambiguous. Months and years are not: they have 28 to 31 days, and leap years add a 29th of February every four years (with the century exceptions). This calculator takes a start date and a mix of years, months, weeks and days, and returns the resulting calendar date plus its day of the week.

The order of operations matters. This tool applies years and months first, then weeks and days. When the start day does not exist in the target month — 31 January plus one month, or 31 March minus one month — the day is clamped to the last valid day of that month rather than spilling into the next one. So 31 January + 1 month gives 28 February (29 in a leap year), which is what most people mean by "the same day next month".

Weeks and days are then added as a plain count of calendar days, which automatically absorbs any 29 February in the span. That is why adding "1 year" to 29 February 2024 produces 28 February 2025: the day arithmetic lands on the real next date, and 2025 has no 29th.

Common uses: contract and notice periods ("30 days from signing"), warranty and return windows, project milestones, prescription refill dates, and trivia like "what day of the week is 10,000 days from my birth". For the number of days between two dates you already know, use the Date Difference Calculator instead — this tool is for projecting a date you do not yet know.

Dates are handled as calendar dates at local midnight, so there is no timezone or daylight-saving drift in the result. Everything is computed in your browser.

result = startDate
  + sign × (years, then months)   // day clamped to last valid day of target month
  + sign × (weeks × 7 + days)     // counted as calendar days, leap days included
sign = +1 for "add", −1 for "subtract"