Adding $date->class

Hi, everyone! I was doing some work for a client who is using Calendar Block to provide availability information. There are two states of availability, partially booked and fully booked. In order to display them, I added a "class" value to $date so that our custom module could set classes for each day in the calendar.

I'm somewhat new to patching, so this may be sub-par or not a good idea at all. Tell me what you think. Thanks!