How to Use This Tool
Counting days between two dates has one genuine complication, and it is not the arithmetic. It is whether the answer should include both ends. Monday to Friday is four days if you are counting the gap and five days if you are counting the days themselves, and both answers appear in real contracts.
The inclusive question
Leave the checkbox off for the gap between two dates. That is what you want for rent periods, interest accrual, notice periods and anything measured in elapsed time. Tick it to count both dates as full days, which is what you want for annual leave, hotel nights described as days, conference length and most legal deadlines that say "within N days".
The distinction sounds pedantic until it is a day of holiday allowance or a day of interest. If a contract does not specify, the convention in most jurisdictions is that a period "of N days" excludes the starting day and includes the last — which is the unticked behaviour here.
The years, months and days breakdown
Beneath the total you get an exact calendar breakdown — "2 years, 3 months and 14 days" — which is what people actually mean when they ask how long ago something was. This is not simply the total divided by 365, because months have different lengths and leap years exist. The calculation walks the calendar properly, so the result matches what you would count on a wall planner.
Working days
The working-day count excludes Saturdays and Sundays. It does not know about public holidays, because those differ by country, by region and by year, and a tool that guessed would be confidently wrong. Subtract your own — most countries have eight to twelve a year, so a 90-day period typically loses two or three beyond the weekends.
Adding days to a date
The bottom section runs the calculation in reverse: give it a start date and a number, and it tells you where you land. Switching to working days makes it skip weekends, which is what "delivery within 10 business days" actually means and is surprisingly annoying to count by hand across a month boundary.
