This calendar extends infinitely into the past and future. It will tell you the day of the week for any date. It allows you to convert between Julian and Gregorian dates, and to calculate the number of days between any two dates, among other things.
Days are identified by their date, expressed in the Julian and Gregorian date systems, and by Julian Day Number. Each day has its own page accessible through a simple URL scheme. For example:
Today, UTC: /cal/gregorian/2010/07/30.
10,000 days before today: /cal/gregorian/2010/07/30-10000.
23rd April 1616 in the Julian calendar (Shakespeare's death) : /cal/julian/1606/4/23.
Julian Day Number 0: /cal/day/0.