when to activate jr pass

Hello, I will be traveling to Japan in January. During the first week I won`t be using the Jr. Pass, since I´ll only be staying in Tokyo.

Then Ill be visiting Korea and I´ll be returning to Japan with a ferry from Busan a week later on the same day that I was hoping to use my Pass. I was hoping to exchange the Jr Pass during my first week and schedule the activation date for my second Japan entry. But now I`m doubting that. I think that I need the last visitor entry to the country? or the first entry stamp will work just fine? Thank you in advance for all your help.