Exchange Order Activation.

Hello,

A pleasure to greet you, I will be arriving in Tokyo on Monday, March 27, 2023. My question is? How far in advance should I buy the 21-day Japan Rail Pass? I know that I must change it within 90 days from the receipt of the change order, are these 3 months or 90 days continuous? For example, I receive the change order on January 15, do I have until April 15 to change it?
The other day on March 27 that I arrive in Tokyo? Can I change the order that same day and activate the pass on the same day, March 27? To travel immediately to Kyoto.

Regards.
Felipe.