Do you have the feeling the working days pass and you have no idea what you achieved? Many separate tasks that take up time but are not easy to identify and group.
The idea for this app is at regular intervals to ping you asking what you have done so far. You control the granularity of it, so you can report anything that takes more than 15 minutes or at least an hour. At the end it generates graphs for you.
Well, basically I just wanted to build an app :)
But the problem with calendars however is that first of all you don't have calendar entries for everything you do, neither those entries stay up-to-date while the day is progressing.