choosing activation date

Hello, I've been trying to figure this out and seem some confliction informaiton online.

Can you clarify please: Do I need to choose my activation date when purchasing the ticket online (it asks me for a date of use), OR can I choose to start the use date when I go to the Exchange order office with the paperwork I've received online? Because I don't know exactly when the item will arrive in the mail, I'm sending it to an address in Japan. Thanks in advance