Add or Subtract Days
A date ± days — the exact date it lands on, weekday included.
How this is calculated
Day arithmetic on the real calendar — leap years included, no "30-day month" approximations. Days are the unambiguous unit (months vary in length, which is why "one month later" starts arguments and "30 days later" doesn't). The everyday uses: notice periods, visa windows, returns deadlines, medication courses, and "90 days from signature" contract clauses.
Adding days, weeks and months are three different operations
Adding days is straightforward: the calendar advances, leap years handled, no ambiguity. Adding weeks is just adding sevens, so the weekday is preserved — which is why "in three weeks" is easier to plan around than "in 21 days" even though they are identical.
Adding months is where it becomes a matter of convention, because months are not a fixed length. One month after 31 January has no obviously correct answer. Two behaviours exist: clamping, which gives 28 February (or 29 in a leap year), and overflowing, which gives 3 March. This page clamps, as most date libraries and most people's intuition do. The consequence worth knowing is that the operation stops being reversible — 31 January plus a month minus a month returns 28 February, not the date you started from.
Deadlines: the day you start counting
"Within 30 days" almost always means 30 days after the trigger date, not counting the trigger day itself — but "almost always" is doing a lot of work in that sentence, and different legal systems and contracts define it differently. Some count from the day of service, some from the following day, and some exclude the day the deadline falls on if it is a non-working day, pushing it to the next business day.
The practical advice is to calculate the earliest defensible date and work to that. If a deadline could be read two ways, being a day early costs nothing and being a day late can cost the matter entirely.
Business days, and why nobody's rule is the same
A working-day count needs three definitions before it means anything: which days of the week count, which public holidays are excluded, and whether the start date counts. Saturday and Sunday are not the weekend everywhere — much of the Middle East uses Friday and Saturday, and some countries run a Saturday half-day. Public holidays differ by country and frequently by province or state.
Because of that, a general-purpose calculator can only offer weekend exclusion honestly. Anything more specific has to come from your own calendar, and the safest method for a short critical span is still to count it on an actual calendar with your local holidays marked.
Common uses
Notice periods, payment terms such as net 30 and net 60, statutory response windows, warranty and returns windows, prescription renewals, gestation and incubation dates, project milestones counted forward from a kick-off, and the 90-day limits that appear throughout visa and residency rules. In every one of these the arithmetic is trivial and the convention is the thing that bites — so establish whether day zero counts before you rely on any date.
Frequently asked questions
What is 90 days from today?
Set the start to today (the default) and days to 90 — the result shows the exact date and its weekday. Month lengths and leap years are handled by walking the real calendar, so the answer matches what a diary would give you.
Can I subtract days instead?
Yes — flip the Add/Subtract toggle, or think of it as negative days. "45 days before the deadline" is exactly what the subtract direction answers.
Why does the weekday matter?
Because deadlines that land on weekends usually roll. The result always names the weekday, so a "payment due in 30 days" landing on a Sunday is visible immediately, before it surprises anyone.
Does it handle month ends sensibly?
Yes — adding days is unambiguous (unlike adding "months"): 30 January + 5 days is 4 February, full stop. That is why contracts count in days, and why this calculator does too.