Inclusive or exclusive?
This is the question that makes two people get different answers from the same dates. Counting from 1 to 3 March is 2 days if you measure the gap, and 3 days if you count the days themselves. Contracts, hotel stays and notice periods each use their own convention, so the checkbox lets you match whichever one applies.
Why "months" is always approximate
Months run from 28 to 31 days, so there is no fixed number of days in one. The years-months-days breakdown works by walking the calendar rather than dividing, which is why it is exact while a "months" figure alone never can be. From 15 January to 16 August 2026 is 213 days — 7 months and 1 day, or 30 weeks and 3 days.
Business days
The business-day count excludes Saturdays and Sundays only. Those same 213 days contain 152 business days and 61 weekend days. Public holidays are not deducted, because they differ by country, state and sometimes industry — if you need a working-days figure for a contract, subtract your own holiday calendar from this number.
Leap years
A year is 365 days except when it is 366. The rule: divisible by 4 is a leap year, unless divisible by 100, unless also divisible by 400. So 2000 was a leap year and 1900 was not. Any date arithmetic done by multiplying years by 365 drifts by about a day every four years — the same trap that catches age calculations.
If you are measuring a person's age rather than a gap, the age calculator gives the answer in the years-months-days form people actually use.