What is the definition of a “day” – 24 hours from the time I opt-in or a 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 11:00 PM, your first day will be counted from 11:00 to 11:59 PM, and the next day will start at midnight (00:00 hours).