Keeping track of military seatime

One of my grandsons has joined the army and will soon start his marine training at Fort Eustis. I’m curious what is the best format for him to keep track of his seatime and training that would pass muster with the NMC later on? I’ve heard stories of guys having their seatime disputed after they retired from the army.

ARMY’s Deck Officer school is located at Ft Eustis. If he pursues their marine transportation program he’ll leave with a ticket.