How Legal Deadlines Are Actually Counted
Legal deadlines are counted in months and years far more often than in days, and month arithmetic is not as simple as it looks. The awkward cases all involve the end of a month, and they are worth understanding before you rely on a date.
General information, not legal advice. Limitation periods, notice lengths and their counting rules are set by local law and by the document in front of you. Nothing here tells you what your deadline is.
The clamping rule
Add one month to 31 January and there is no 31 February to land on. The universal convention, and the one the statute of limitations calculator follows, is to clamp to the last valid day of the target month — so 31 January plus one month is 28 February in an ordinary year.
The same rule handles leap days. 29 February 2024 plus one year is 28 February 2025, because 2025 has no 29 February. Add four years instead and you land back on 29 February 2028, since that year does.
A month can be shorter than thirty days
This is where it becomes practical. From 31 January:
- one month → 28 February, a span of 28 days
- thirty days → 2 March, a span of 30 days
- four weeks → 28 February, a span of 28 days
“One month” and “30 days” are four days apart from the same start date, and which one applies is a question about the wording of your contract or statute, not about arithmetic.
Months do not accumulate evenly
Because each month has its own length, a multi-month period is not a multiple of a single month. From that same 31 January:
- one month → 28 days
- two months → 31 March, 59 days
- three months → 30 April, 89 days
Nothing here is a rounding error; it is just the calendar. If you need a fixed number of days, the document has to say days.
Two start dates, one deadline
A consequence of clamping that catches people counting backwards: 30 January and 31 January both give 28 February when you add a month. Working forwards this is harmless. Working backwards from a known deadline to reconstruct when a clock started, it means the answer may not be unique.
If the start date matters — and for a limitation period it matters a great deal — establish it from the record rather than by reversing the arithmetic.
The deadline is the last day, not a target
Whatever the arithmetic produces, treat it as the outer boundary rather than the plan. Filing requirements, court opening hours, holidays and rules about what happens when a deadline falls on a weekend can all move the practical last day earlier than the calculated one.
Several jurisdictions have rules extending a deadline that falls on a non-business day; others do not, and the ones that do differ on what counts. A calculator has no way to know which applies to you.
What none of this settles
The arithmetic is the easy half. The hard half is which period applies to your claim, when the clock started, and whether anything paused it — and those are questions of law that vary by jurisdiction and by the type of claim.
Use the arithmetic to know roughly how much time you may have, and use that answer as a reason to get advice promptly rather than as a substitute for it.