Parallax View - Date-Subtraction Calculator
Subtracts two Gregorian dates and returns the number of intervening days. A special calendar control makes selections of the dates easy.
Rosetta Calendar - A Calendar Conversion Service
Scott Lee's free on-line service for converting to/from dates in Gregorian, Julian, French republican and Jewish calendars.
The Gregorian Conversion | Polysyllabic
Background information from A History of the Western Calendar.
Tom Nunamaker's Census Age Calculator
For UK census years.
Calendars & Dates » Calculators & Converters