Guides
Deadlines, Notice Periods and Date Arithmetic
How to count a notice period backwards, what “one month” means in a contract, how month-end and leap days break naive arithmetic, and how to record a deadline defensibly.
Updated: 2026-10-19 · 6 min read
Most date calculations are forward: given two dates, how long between them. Contract work runs the other way. You are given a deadline or a notice period and need to know the date it lands on — and that is where the awkward cases live.
Calendar days and business days are different promises
“Within 30 days” and “within 30 business days” are not close to the same thing. Thirty business days is about six calendar weeks, and the gap widens whenever a holiday falls inside the range.
Before calculating, find the exact wording. Courts and arbitrators read these phrases literally, and a deadline computed on the wrong basis is usually not excused by the mistake.
Counting a notice period backwards
To find the last valid day for notice, start at the effective date and subtract. Subtract whole months first, because “three months' notice” means the same day number three months earlier, then adjust for weekends and holidays if the agreement requires a business day.
If the resulting date does not exist — 31 November, or 29 February in a common year — the usual convention is the last day of that month. That rule is common in law but not universal, so it is worth checking the governing contract.
What “one month” actually means
A month is not 30 days. Adding one month to 15 January gives 15 February; adding 30 days gives 14 February. The difference compounds over a year: twelve 30-day additions come to 360 days, five days short of a calendar year.
Month addition has a second trap. Adding one month to 31 January has no obvious answer, because February has no 31st. Most systems clamp to the last day of the target month, giving 28 or 29 February, but some roll forward into March. Two systems that disagree by a day will disagree about whether a payment was late.
Leap days in long agreements
A one-year term starting on 29 February 2028 ends on 28 February 2029 under the clamping rule, because 2029 has no 29 February. A system that rolls forward instead would say 1 March, which is a day late.
Over a multi-year term, leap days also affect day counts: the same nominal term contains one more day if it includes 29 February. Interest calculations that use an actual/365 convention will notice; those using actual/actual will not.
Recording a deadline defensibly
The arithmetic is the easy part. What causes disputes is not being able to show how the date was reached. A deadline that can be defended records four things.
- The rule as written, quoted exactly, including whether days are calendar or business days.
- The start date and whether it counts — “from the date of this notice” is usually the day after.
- The calendar used for business days, naming the country and any holidays removed.
- The computed date, the method, and the date the calculation was made.