Early check-in means for me that you are not supposed to have a night just after check-in (if not, this is a late check-in...).

The sooner was 7am for me (at IC)
The later was 2.30 am (but it was a late check-in, not an early check-in).

I always inform hotel of late arrivals (after 9pm) to make sure the room is not given to someone else.