Counting days by hand runs into two things people usually get wrong: leap years, and whether the start or end date is inclusive. This counts the exact number of days between two dates, adds or subtracts time from a date, calculates an exact age, and counts business days while skipping weekends.
When you'd use this
Working out a project deadline that's "10 business days from now," calculating someone's exact age for a form, or checking how many days remain until (or have passed since) a specific date.
Common errors
An off-by-one result almost always comes down to inclusive vs. exclusive counting. "From Monday to Friday" is 4 days later, or 5 days if you count both endpoints. Check which convention the context you're working in expects.
Business-day counts exclude weekends, but they know nothing about public holidays, since those vary by country and by region. Factor those in separately where they apply.
Frequently asked questions
Does the business-day calculation account for public holidays?
No. It skips Saturdays and Sundays only, since holiday calendars vary by country and even by organization. Where specific holidays apply, subtract those days separately from the result.
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