I became interested in Julian Dates after investigating the issues surrounding the impending Y2K computer software problems. The Julian Date system indicates the number of days since January 1, -4712, i.e., January 1, 4713 BC/BCE. It was proposed by Joseph Justus Scaliger (1540-1609) in 1583 (with this system named for his father, Julius Caesar Scaliger). Day One is a day when three calendrical cycles converge (the 28 year Julian Calendar, or Solar Number; the 19 year Golden Number cycle; and the 15 year ancient Roman tax cycle, called the Indiction). Scaliger picked January 1, 4713 BC on the Julian Calendar as Day One. The three cycles coincide every 7980 years.
This applet/application makes use of the JulianDate class which assumes that the switch over date (from the Julian, or Solar, Calendar to the Gregorian Calendar) occurred on 15 Oct 1582 (the day following 4 Oct 1582). For those with further interest you might want to check out the basic algorithm used in doing such conversions or check out this short history of calendar usage.
To use the applet, just change the desired text field. The other text field will be updated when you hit return/enter.
This applet/application has been written to be an applet (as seen above) and an application rolled up into the same code. As an application either a Julian or Gregorian Date can be given on the command line and the equivalent alternate date is printed. If no command line arguments are given, then today's date is printed as a Julian Date. If the single command line argument "-applet" is given, then the application presents itself in its own window appearing as the above applet does.