Calendar items can have email reminders at several offsets, such as one week before, one day before or on the day.
New reminder times default to 7:00am. Each selected reminder can now have its own send time, so a one-week reminder could arrive at 9:00am while a day-before reminder arrives at 6:00pm.
Nudges are deliberately simple reminder emails. Completed, cancelled or otherwise inactive items do not keep sending normal future reminders.