Time & Date Calculators

Date arithmetic looks trivial and is not. Months have no fixed length, so "three months from November 30" has more than one defensible answer; leap years break naive day counts; and a business-day calculation is only as good as its holiday list.

These tools follow ISO 8601 where it applies, state the rule they used where it does not, and work in whole calendar days rather than in milliseconds — which is what stops a duration from being off by one every time daylight saving moves. The holiday list behind the business-day tool is US federal, generated from the rules rather than a hard-coded table, so it stays correct in future years.