Everything on this site, in one list
Sixteen hand-built calculators, ten indexes of worked-out answers, and the pages that explain how the numbers are produced. The generated answer pages — several hundred of them, covering the day, week, month, year, business-day, holiday and time-zone counts people actually search for — are reached through the indexes below rather than listed here.
If you are trying to decide which tool you want: use a calculator when you have your own dates to put in, and a worked-out answer when you want the number for a common count without typing anything. Both use the same rules, which are written out on the methodology page.