How to have some appointments start at 0X:15am some days and others 0X:00am?

My client requires appointments for a service provider to be staggered like this:

1) Monday they start at 9:15am and are 30 minutes in length each throughout the day

2) Tuesday they start at 10am and are 30 minutes in length each throughout the day

3) Wed/Thur/Fri varies (some weeks the start is 9:15am and some weeks 10am).

Is there any way to accomplish this? I've set the "time base" for 30 and the "additional time base" for 15 and selected each but, for some reason, when I select the 15 the appointments are suddenly 45 minutes in length?

Scratching my head and can't figure it out yet.