Counting days between dates by hand is unreliable for a specific reason: months have different lengths, February changes with the leap year, and it is easy to be off by one depending on whether you count the start date, the end date, or both. Contracts, notice periods and deadlines usually depend on getting exactly that detail right.
The convention in most legal and commercial contexts is to exclude the start date and include the end date. A thirty-day notice given on the first of the month expires at the end of the thirty-first day counted from the second.
Leap year rules
- A year divisible by 4 is a leap year.
- Unless it is divisible by 100, in which case it is not.
- Unless it is also divisible by 400, in which case it is. 2000 was a leap year; 1900 and 2100 are not.
- The rule exists because the solar year is 365.2422 days, not 365.25 — the correction keeps the calendar aligned to within one day every 3,000 years.
Useful day counts
| Period | Days | Notes |
|---|---|---|
| One week | 7 | 5 working days |
| One month | 28–31 | Averages 30.44 |
| One quarter | 90–92 | |
| One year | 365 | 366 in a leap year |
| One decade | 3,652 or 3,653 | Depends on leap-year placement |
Calendar days versus business days
A "30 day" term almost always means calendar days; "30 business days" is roughly six weeks. The distinction is worth confirming in writing, because the gap is substantial and both parties tend to assume the reading that suits them. Business-day counts also depend on which public holidays apply, which varies by country and sometimes by state — a detail that matters on international contracts.
Common uses
- Contract and notice periods, where the exact expiry date has consequences.
- Project planning and milestone spacing.
- Age in days, used in paediatric medicine and in some visa applications.
- Countdown to an event — days until a wedding, a launch or an exam.
- Interest accrual on a daily basis, common in short-term lending.