Setting Work Schedule with Outlook an half hour boundaries
That is a limitation of the Outlook.

I use a work around.

I have set up a repeating event 7-8:30 for every work day, and marked it as "out of office" status, and Private and named it "do not disturb".

At least in the view now my work day starts at 8:30, and no one can call me in a god forsaken way to early meeting.