Is the travel period included in the 3 months after issue date?

Hi there,

I am just wondering with purchasing the JR Rail Pass with the intention to use it in Early Jan for 3 Weeks will I still be able to purchase it before the price increase or will I miss the cut off?

Reason for my question is that I am interpreting it as, I can purchase it 3 months prior up until the end of September and then i will have 30 days to use it upon activation.

I will be arriving in Japan mid-Decemeber, so am I able to purchase in Sep activate it upon arrival for the intended travel date being early Jan?

Thanks in advance for any help :)