Most of you will remember that right at the start of this year Apple was found to have deliberately throttled older iPhones, with the official reason being it was necessary thanks to issues face by older batteries that affected performance. As part of a sorry campaign Apple offered iPhone users a one-time-only, no questions asked, cheap battery replacement. That means people pay around £25 instead of £79.
The thing about this offer was that there were some catches involved, and the main one was that this offer was only valid in 2018. As The Guardian pointed out earlier today, this means people who haven't taken advantage of this deal only have 12 days to get their act together. Assuming you need your phone's battery replaced, that is, otherwise you're just wasting your own time and money for something trivial.
This offer is designed for phone affected by throttling that are no longer covered by Apple's warranty. That means anything as old as the iPhone 6 (including the SE) and as new as the iPhone X are eligible, though last year's phones are far less likely to have any of those same problems. They also may still be covered by the initial warranty (or AppleCare+), which means a battery replacement is free - regardless of whether you get your phone in before the New Year.
You can start a battery replacement over on Apple's support website, and you can opt to pay an extra £7.44 to post your phone and save yourself a trip to the Apple Store. After deadline you'll end up having to pay £45 for an out of warranty replacement (£65 for the iPhone X), and while that's better than £79 it's not as good as £25.
So what are you waiting for? You've dawdled long enough.