These are taking advantage, for years I managed clients who wanted sliding around start times but it was not fair. I put my foot down making it clear it was set time or no booking, and from then on they made a booking for a specific time knowing if they were late they would not get an overrun and or may have the time subtracted. Unfortunately one of these also went on to push other boundaries one day stepped far to far so I refused to see him again. These days if anyone asks for a sliding start I only accept the furthest time of their slide, no one has had a problem with me stating I only work to set times.
Overruns I have experience of many, I need a lot of time between bookings and or whatever else I am doing. I clean and tidy thoroughly, have emails, texts, phone calls to answer, as well as my own maintenance. Having less than plenty of time is energy sapping and means I'm running around like an idiot. If they can only do a time that suits them, then they have to accept I may have to finish on time. Also to expect more is completely unfair, extensions cannot be guaranteed depends on my arrangements - I plan based on the booked time.
From past experience the only punters, clients, regulars who have problems with finishing on or just after time are just not worth it. There'll be others who will be timely, considerate and be ready to go without much fuss being very happy with the full time they paid for without feeling the need to hang on.