Calendar days vs working days — when each matters
For legal deadlines, loan terms, and contract notices, calendar days are almost always what the law means. '30 days' in a lease typically means 30 calendar days from the notice date.
For work projects, delivery estimates, and anything measured in business time, working days are the practical unit. A 10 working-day delivery isn't two weeks if there are two weekends in the window — it's 14 calendar days. Many shipping carriers quote working days, as do courts for certain procedural deadlines.
Why February breaks everything
February causes most date arithmetic edge cases.
- One month from January 31 could mean February 28 (or 29 in a leap year) — most systems land on the last day of the target month.
- Adding 30 days to January 31 gives March 2 (or March 1 in a leap year) — different from 'one month later'.
- Leap years occur every 4 years (divisible by 4), except centuries (divisible by 100) — unless also divisible by 400. So 2000 was a leap year; 1900 was not.
- The formatted breakdown (years + months + days) in this calculator follows the 'days remaining in the partial month' rule — the most natural human interpretation.
Common date difference questions answered
A few frequently needed calculations to sanity-check the tool.
- Days in a year: 365 (or 366 in a leap year). Days in a month: 28–31 depending on the month.
- Days in a decade: 3,652 or 3,653 (depending on how many leap years fall inside).
- Working days in a year: approximately 260–262 (52 weeks × 5 days ± 1–2 day adjustments for year start).
- Days between two dates exactly 1 year apart (same month/day): always 365, unless a Feb 29 falls between them, in which case 366.