Got thinking I might write one for the android platform. Something quick and dirty.
I know what I would do for myself. On first run I would allow the user to create a database with things like instructor's names and class names (if they wish to use them). A basic app would be to have it start up asking if you want to log in a new session. So when you click "start up" it will ask you to either accept the current time or adjust it accordingly. You select the class name and sensei, filling in if necessary. Screen changes to a timer showing time in. I suppose I could put up a pause timer button at that time along with a button saying end session.
On end you can adjust the timing (4 hours later you realize you forgot to close it out -- oops). Then they have the option to save the session.
Options I would have for myself would be to show total hours. Then allow a report on hours by class title or hours by instructor. I suppose I'd need to allow for all those reports since a selected date.
Anyway, just my ideas. I was thinking of doing it simply as an intellectual exercise. In our dojo all instructors take attendance and there is a running total available of hours/year and hours since last exam.
Any other ideas welcome. Not sure I'm gonna write the app, but I was thinking about it.
a site like martialmanagement.com, with a mobile page (which is in development I believe) does all that and is platform (android, ios, win7p, blackberry) independent.