Adjacent feature request to grouping. I'd love to be able to add a third level for grouping Days, Tasks and then Tags.
I also use WorkingHours to track my time before submitting to another enterprise application. Grouping by day makes it easy to sum up all of the smaller chunks of time I worked on a project throughout that day. By task provides me the ability to tell which project, but adding an additional (3rd) grouping option for tag would help to define what time inside of that project was used for design, dev, test, support, etc.
Currently I'm exporting and grouping in Numbers or Excel to collect this data. It'd be nice to be able to do this in WorkingHours to use immediately in SubTotal.