Do I get bonus days for renewing early?

Renew the same plan while it's still active and your remaining days carry over in full, plus 2 bonus days on top of what you buy.

Buying a different plan isn't a like-for-like renewal: your remaining days are converted by value first (see below), so you can end up with fewer days than you started with. Extending with SonicCoins, and subscriptions billed by Apple or Google, don't include the bonus days.