As YouTuber JerryRigEverything explains in the video below, repairing an iPhone maliciously hit by the iOS date bug isn’t actually that hard. You’ll need specialist yet inexpensive pentalobe screwdrivers, but the whole procedure can be completed literally within minutes. It involves simply lifting the screen, unscrewing the battery connector protection plate, disconnecting the battery, and then repeating these steps in reverse.
Of course, the simplest solution is to either take the device into an Apple Store for them to remove the battery, or to let the battery run out of juice of its own accord. However, for various reasons either of these two options might not be feasible – you might live a million miles from the nearest Apple Store, for example, and a battery with near full charge can take days if not weeks to drain if not in active use.