Think of it like check in time at a hotel. No earlier than 12. Just make sure you are there before the all-aboard time! They will leave without you!
No worries. Show up at port between 11am and 3pm. Don't stress about arrival time. The only penalty is if you arrive toooo late and miss the ship. It's all good. Some feel they need to board early, and some don't. From recent reports it seems the sweet spot for arrival is 12:30 - 1:00. Which ship, port, and month are you sailing? Yes, if you arrive before or after your "Port Arrival Time" , you walk in the terminal and check in. No worries.
As noted, there is no problem at all with arriving after your scheduled arrival time provided you are there before "all aboard" time. If you arrive before your scheduled time, there is the possibility of needing to wait a short time before entering the terminal (this happens only on the new ships). The wait is typically not more than 10-15 minutes. If you arrive after boarding starts, no problem at all. They just can't let the number of guests in the terminal exceed the fire marshal regulations.
