You just set the time and date you want as time zero and whether you want to count up or down and off it goes. You can set font size, font and background colour, and decide in which order you want your counts displayed. You can also set whether you want to display just whole days, or "millidays" or even 1/10000ths of a day.
This sounds neat though, doesn't it? So just download D-Time now.
D-Time could stand for Decimal Time, Diurnal Time or Day-Based Time, I personally prefer Decimal Time. D-Time is a date/time system based on counting days and days only. Time of day is given as a fraction of a day indicating how much time has passed since the date-change. Decimal Time Global uses as time zero July 13th 1903 00:00 UTC (Universal Time coordinated).
You can read more about the D-Time standard in NAITA001-0 and you find the instructions for downloading and installing the D-Time application here. The earlier version of this page, with its rant against the ordinary time keeping system can be found right here.
If you don't want to download the application, or you don't want to install the required Java 2 platform, you can play with these three java-script pages. Convert Time for converting time of day into fractions of a day, Convert Date for converting from a gregorian to decimal dates, and Counting Days for giving you the number between two dates, or a date a certain number of days in the future.