To add to robo, I think in the case of renewals you need to activate it by jan 3rd as it's guest relations/ticket booth that adds the extra month to the expiration. Only new passes puchased in the window have 18 months to activate them..
@ravenclawtrekkie might be able to offer insight if that is accurate.
Do the math, I'm not sure renewing is going to save you money if you can't get that 13th month. It might be better to buy a new AP before jan 3, but not activate it until your april trip and then you'll get a may 2018 renewal date. You can buy an AP even if you have an AP. there's a few months there that I think is more than 15% of the cost that your never going to use it. at least with a may 2018 you might get more use out of it.