I have been using Brett Terpstra’s Slogger for a number of years to update Day One with information from services I use. One thing that was missing was keeping some sort of record of the tasks I had completed in a day, in an easy to view fashion.
I use The Hit List as my task management application of choice, after switching back from a Things/Reminders mismatch solution. I originally got the Mac version of this application through a MacHeist promotion, but had switched away from it around the time that the iPhone application was released (after much delay) which required a paid subscription for sync. This sync service is now free, so I gave the application another chance and I am hooked again.
Based on the Things plugin which is included in the Slogger project, I created a plugin to capture the completed tasks from The Hit List and group them by date.
The majority of the code in this plugin is Applescript. I had written this part separately before making the small changes required to run this embedded in a Ruby script and parse the output. It is definitely not an efficient way of finding the completed tasks, as for each date in the filter range it effectively does a depth first search looking for tasks completed on that day, but it achieves the goal.