Can I start my meeting before the scheduled start time?
Yes. Your meeting becomes available to you immediately after you schedule it, so you can start it any time you'd like. Starting a meetingearly (whether by accident or on purpose so you could do a practice run through) does not affect your schedule or invitation in any way, so don't worry.
Note: Keep in mind that if you scheduled a recurring meeting, you can use it at any time and it never expires!