On the Chronosight Blog, there's a follow-up post with a look at some enhancements to the original tutorial - Simple Calendar Tutorial.

Almost a year ago I made a tutorial on Simple Calendar Tutorial which trained us to develop a calendar using PHP. Now it times to enhance that tutorial to a whole new level. Let trou in some AJAX code and Javascript Document Object Model (DOM) to create not one, but three type of calendar: Single Calendar, Time Period Selection Calendar, Month-Year Calendar

He steps you through the creation of two files - one to provide a space for the Ajax interface and another for the calendar PHP class. Toss in a little CSS and some explainations along the way and you have all that you need to make a simple, easy to use calendaring system.