Julian Day Numbers
http://www.hermetic.ch/cal_stud/jdn.htm (All about JDNs)
http://www.numerical-recipes.com/julian.html (A JDN calculator)
Julian Day Numbers are a dating system adopted by the astronomical community and having its roots in work done in the sixteenth century. "Days" are counted from a rather arbitrary start on January 1st, 4713 B.C. To know the number of days elapsed between two dates it's only necessary to subtract one JDN from the other. The first site above has a detailed discussion of Julian Day Numbers, their history, their complexities and their applications.
The second link above has a neat, user-friendly calculator for obtaining the JDN for any two calendar dates and even displays the number of days in the interval in between. (You can download the Java Script that operates the calculator and install it in your computer or server to be operated by your own browser.)
This calculator yielded 1116 days between 13 July, 2006 and 23 June, 2003. The hit counter read 3953 at the time I opened the page giving an average of 3953/1116 = 3.54 hits per day.