What is Considered a day; From opt-in or Calendar day?

We define day as a calendar day, i.e., your daily limit is applicable from 00:00 hours to 23:59 hours, irrespective of when you enable it.

Therefore, if you opt-in at 23:00, your first day will be counted from 23:00 to 23:59, and the next day will start at midnight (00:00 hours).