Thirty days from 31 January 2026 is 2 March. One month from the same date is 28 February. Both are ordinary readings of "a month later", and a contract that says thirty days means the first while a tenancy that says monthly means the second. That is why notice periods are written in days wherever the exact date has to be provable.
Adding days is unambiguous arithmetic — no month lengths, no clamping. Adding months keeps the day-of-month number and clamps when the target month is too short, which is a convention rather than a fact.
Which form does each situation use?
The pattern is that anything requiring a provable date uses days, and anything recurring uses months.
| Period | Written as | Why |
|---|---|---|
| Payment terms | 30, 60, 90 days | Must be countable without a calendar argument |
| Returns window | 14 or 30 days | Same |
| Statutory notice | Days, often working days | Provable, and courts count them |
| Tenancy or subscription | Calendar months | Recurs on the same date each time |
| Interest accrual | Days over a day-count basis | Fractions of a year, not calendar months |
A hundred days from 1 January 2026 is 11 April 2026, a Saturday — which is the other half of forward counting that people forget until the date arrives and nobody is in the office.
Does the start day count?
Usually not. Periods generally begin the day after the triggering event, which is why "within 30 days of receipt" gives you thirty full days and not twenty-nine plus a fraction of the day the letter arrived.
Where it genuinely matters, drafting says "clear days" to make the exclusion explicit at both ends. That phrase exists precisely because the default is contested often enough to need a name.
The inclusive-or-exclusive article covers the same distinction from the measuring side.
Why count backwards?
Because that is what deadlines actually require. A filing due on a fixed date with a 21-day notice period needs the date 21 days before it, and being a day out is the difference between served and not served.
Backwards counting works identically: thirty days before 2 March 2026 is 31 January, and subtracting one month instead gives 2 February. The asymmetry is worth noticing — subtracting a month from 2 March and adding a month to the result does not always return you to 2 March, because clamping is not reversible.
Does the day of the week come free?
It does, and it is worth reading. Any date arithmetic that lands on a specific date also lands on a specific weekday, and that weekday decides whether the deadline is workable.
The pattern is regular enough to check in your head: a non-leap year advances the weekday of a given date by one, and a leap year by two. That is why your birthday moves forward a day most years and two after a February with 29 in it.
What about working days?
A different count again, and the one most likely to be assumed silently. Ten working days from a Monday lands on the Friday of the following week — fourteen calendar days later, which is nearly double.
Whether the starting day counts is a genuine convention difference. Most service agreements and legal notice periods start from the next working day, so day one is the first full working day after the trigger rather than the trigger itself.
Public holidays are the part no calculator can settle for you, because they differ by country, by region and sometimes by industry. They have to be deducted deliberately, which is why a working-day figure is only as good as the holiday list behind it.
What is a day-count basis?
The convention interest uses to turn a period into a fraction of a year, and there are several. Actual over 365 counts real days against a 365-day year. Thirty over 360 pretends every month has 30 days and every year 360, which makes bond coupons identical and slightly misstates every period.
It matters because the same loan over the same dates produces different interest under different bases. On a large principal the gap is material, and it is the kind of detail that lives in a schedule nobody reads until the final payment does not match.
Questions people ask
Is thirty days the same as a month? Only in February, and only in a non-leap year. Across a year the two drift by five days.
What if the deadline lands on a weekend? Many rules move it to the next working day, and a few move it back to the previous one. It is one of the details worth checking in the wording rather than assuming.
Does adding a month to the 30th of a month work? Yes, until the target is February. 30 January plus a month clamps to 28 February, and the leftover days are lost rather than carried.
Why is the day of the week worth knowing? Because a date that falls on a Saturday is effectively the following Monday for anything requiring a human. Forward counting that ignores the weekday produces deadlines nobody can meet.
Count in days when the date has to be provable and in months when the obligation recurs. Add days to a date and subtract days handle the two directions, date plus days is the unambiguous fixed-count version, and the date calculator covers all three shapes of the question at once.