Nothing shady about it. If my car completes production on December 31 with a 2014 VIN, I take delivery on January 2, and offer it the next day, do I have a year old car or a 2 day old car? Any legitimate buyer would understand the difference, and the fact that Tesla doesn't have a model year makes the in service date all the more important.
You'll likely have the paperwork to show when you took delivery, but I do think it would be shady to list it as a 2015 Model S.
I do agree that it may be all moot, because I've seen several Model S's listed without years. When a potential buyer inquires, you could just state the delivery date. Or you could just state the delivery date within the body of the listing.
Plus, outside of age, years dont really have the same meaning. With a 2014 ICE you typically can see which options and standard features are available. With Tesla they add features when they feel like it (autopilot, heated steering wheel, etc.)