Happy to hear that you've found a new use, journaling, for calendar.txt. I'm using it similarly myself, even though it's not mentioned on the page. For some projects, I collect billable hours from old calendar.txt entries ('grep +theproject calendar.txt'). I also use it to record some results, e.g. Cooper x meters on this date.
Do you use calendar.txt for journaling and another program for calendar planning? If so, what are you using for a calendar?