I'm wanting to do the same thing. Have all my subscriptions expire on a specific date instead of a timeframe. I'm running version 3.8.2 and all I see in the subscription manager is subcription length. Any help is appreciated.
In subscription manager you can edit every subscription and manually change the expiration date to December 31st.
I assume you'll extend people's existing subscriptions since if you shorted a subscription someone already paid for they'd no doubt get upset, and might very well file a complaint with PayPal or whatever e-commerce solution you are using.
The problem is you don't know when someone will renew and you can't force them to buy only on December 31st so every year you'll run into the same issue.