Date math basics
The calculator counts calendar days (end − start) and breaks them into weeks, months and years. Business days exclude Saturdays and Sundays (and optionally holidays you can account for manually). The add-days mode answers "90 days from today is…" — useful for shipping windows, visa waits and project deadlines.
What counts as a day
- Calendar days: every day including weekends — the number you see on contracts and deadlines.
- Business days: Mon–Fri only. 10 business days ≈ 2 calendar weeks; the tool shows both so you can't be surprised.
- Inclusive counting: some contexts (e.g. "3-day notice") count both endpoints. The tool uses exclusive end (standard date-diff); add 1 if your context includes the end date.
- Month/year breakdowns are approximate (months have 28–31 days) — the day count is always exact.
Practical uses
Countdown to a trip or exam, invoice due dates, visa/immigration day counts, "how many school days left", lease notice periods. Pair with the Age Calculator for birthday math and the Working Days Calculator for project scheduling.