By default, all times are assumed "available" in a Google calendar. Only once an event is inserted, the corresponding time slot is considered "busy". It is possible to define working hours, but this is not flexible. Assume you have a freelancer who is available Monday 10:00-14:00 and Thursday 11:00-16:00. And, in the week afterwards, he has different time slots. Is it possible to manually define the working hours?