Need to be able to select any start time for appointment

When you change the time duration in the availability tab - If you say click 3hrs - then the times are more limited so it shows 8am, 11am, 2pm, etc - instead of showing all times and all possible times - eg 10:30 or 10:45 - I need it to show all times for every 15min block..
Basically exactly how it shows on the 15min booking one.