Counting days between two dates sounds completely trivial — until a legal deadline, a notice period, a payment due date, or a probationary period depends on it. The calendar is full of irregularities (months of different lengths, leap years) that make manual counting genuinely error-prone. Here's how to do it right.
Why Manual Counting Goes Wrong
- Off-by-one: is the start date counted? The end date? Conventions vary by context.
- Month-length confusion: February has 28 or 29 days; months range from 28 to 31.
- Leap year blindness: 2024, 2028 are leap years; 2100 is NOT (century years are only leap years if divisible by 400).
Our time between dates calculator handles all this automatically. For age calculations specifically, our age calculator is purpose-built.
Inclusive vs Exclusive Counting
This causes the most confusion in legal and contractual contexts. Does today count as day 1 or day 0? Exclusive: today not counted — 3 days from Monday = Thursday. Inclusive: today is day 1 — 3 days from Monday = Wednesday. UK law varies by context — statutory notice periods and court deadlines each have specific rules. When stakes are high, check the specific wording and confirm with a professional.
Calendar Days vs Working Days
Many deadlines are expressed in working days — calendar days excluding weekends and public holidays. The UK has 8 bank holidays in England and Wales (different in Scotland and Northern Ireland). Five working days from a Monday before a bank holiday Tuesday doesn't land where you'd expect. For critical deadlines in working days, always count manually through the actual calendar for that specific period.
Practical Applications
- Tenancy notice periods (typically 1-2 months)
- Consumer returns windows (14 or 30 days)
- Invoice payment terms (30, 60, 90 days)
- Employee probationary periods
- Event countdowns
Spreadsheet Date Maths
In Excel and Google Sheets, you can subtract one date from another directly to get the number of days: =B1-A1. Format the result cell as a number, not a date. For working days, =NETWORKDAYS(A1,B1) counts business days between two dates, excluding weekends and optionally specified holidays.
For anything with meaningful consequences — legal, financial, or contractual — use a calculator rather than counting on fingers. Calendar maths is just tricky enough to produce one-day errors that turn into real problems.
Further reading: GOV.UK has guidance on legal time limits and deadline counting in civil proceedings. Visit GOV.UK for guidance on legal deadlines.