Just a note to say that my Garmin Colorado now indicates the incorrect date on both the display and when tracks are downloaded. Garmin has issued a statment https://support.garmin.com/en-GB/?faq=z ... Y9kDnDBgZ6 and they have confirmed to me via email "However, the positioning accuracy will not be affected. The device will continue to deliver the same positioning performance as before the rollover."
They also now have a case open and I hope may provide a firmware fix but I'm not holding my breath!!