Date Calculator

Pick a start date, choose add or subtract, and enter how many years, months, weeks, or days to move. You get the resulting date with its weekday, an ISO date you can copy straight into a form, and the total number of days the shift covers. Month math clamps to the end of the month, so January 31 plus one month lands on February 28 rather than spilling into March. Useful for deadlines, contract terms, notice periods, due dates, and anniversaries. It all runs in your browser.

Time

Start date

YearsMonthsWeeksDays

Sunday, 28 June 2026

Same as the start date - add an offset above.

ISO date

2026-06-28

Days from start

What this tool does

It moves a date forwards or backwards by a span you set in years, months, weeks, and days. Set the start date, flip between add and subtract, and fill in the fields - the result updates as you type. The output shows the full date with weekday, the plain ISO form for copying, and how many days the shift adds up to.

How month and year math works

Adding months and years can land on a day that doesn't exist in the target month. When that happens the result clamps to the last day of the month, so January 31 plus one month is February 28 (or 29 in a leap year). Weeks and days are exact - a week is always seven days - and they're applied after the month shift.

Common uses

Work out a 30-day or 90-day deadline, a contract end date, a notice period, a return-by date, or when a subscription renews. Subtract instead of add to find a start date that lands a fixed span before a known deadline.