Sorry, I don't see it as illegal. All changing the date does is let you keep the ebook a little bit longer after your library has checked it out to you. You can't lend it to anyone or give it away while it stays on your reader. Doing this doesn't keep anyone else from being able to check out the ebook. eBooks return automatically from Adobe Digital Editions on your computer at the end of 3 weeks. Your library pays a fee to Overdrive for each time you check out a book and put it on your Sony. Changing the date allows you to read for a few more days just as if you kept a hardback book a few extra days. Our library gives us a 4 day grace period for all hardback books without fines.
Right now when you get audiobooks through Overdrive from your library, it isn't removed at the end of the three weeks by the DRM. As long as it is for your personal use, it will stay on your MP3 Player. You are asked to remove it from your player when finished. This is Overdrive only. If you weren't finished, you would have to go back to the end of the line for the book. Right now several books I am waiting for will be 168 + days before I can get to them again. 8 people ahead of me for 21 days each plus a three day hold. NetLibrary audiobooks disappear at the end of the checkout period, but they do allow you to check it out again right away, so this works better for most readers.