Counting Days: Contracts, Schedules and Age Use Different Rules
"How many days from 1 January to 31 January?" The answer is 30 or 31 — not an arithmetic error, but different conventions. Contracts, schedules, visas and age calculations each default to different rules; if the rule is unstated, disputes follow.
Convention 1: inclusive endpoints
Pure date difference (exclusive of the end date): 1 Jan → 31 Jan = 30 days. "Inclusive of both endpoints" gives 31.
Hotels bill nights, construction counts calendar days, visas count days of stay. Example: 2026-01-01 → 2026-11-02 = 305 days (2026 is not a leap year; Jan–Oct total 304, plus one day into November). Inclusive of both ends: 306.
State which endpoints count in the contract itself — far cheaper than arguing later.
Convention 2: calendar versus business days
Calendar days include weekends and holidays; business days exclude them, then adjust for statutory holidays (and make-up working days).
Payroll uses 21.75 days/month ((365 − 104) ÷ 12), but that is not the actual number of working days in any given month — February may have only 19. Do not use it to schedule a project.
"7 business days" and "7 days" in payment terms, delivery dates or SLAs can be nearly twice as far apart. Always confirm.
Convention 3: borrowing across months
Age and "X years, Y months, Z days" require calendar-wise subtraction with borrowing, not total days ÷ 365.
1990-05-20 → 2026-11-02: 36 years, 6 months, −18 days; borrow from the month (October has 31 days): −18 + 31 = 13 days, months drop to 5 → 36 years, 5 months, 13 days.
People born on 29 February are treated as 28 February or 1 March in non-leap years depending on the institution — follow the receiving authority for documents, pensions and school entry.
Convention 4: time zones and DST
Align on one time zone first, or "the same day" may differ by a day. With timestamps, daylight-saving switch days lose or gain an hour, so 24 hours is not always one day.
For cross-border contracts, name the basis (e.g. UTC+8 or UTC). Store timestamps in UTC and convert only for display — that avoids most problems.
A checklist that prevents disputes
- State whether start and end dates count
- State calendar versus business days, plus holiday and make-up-day rules
- State the time-zone basis
- For "X months later", say calendar months or 30-day periods
- Give a concrete example as an anchor ("e.g. 1–31 January counts as 31 days")
Writing the convention down matters more than which convention you pick.
This article provides general information and calculation methods only. It is not medical, investment or legal advice; consult a qualified professional for your situation.