I have a much older Dallas RTC in a small computer (a cobalt cube2) that I recently plugged in after it had been turned off since its external power supply failed, coincidentally, around 2005, 15 years ago.
The RTC was still running and when I figured in the three hours time difference it wasnt off by that much. (I forget exactly how much though

) . The time servers etc. were not able to update the time since the net has changed a lot since then so I am certain that its still on the wrong time. It would be interesting to figure out the error. Not too shabby.
You definitely should be able to improve on a 15 year old part. Have you thought of energy harvesting electricity somehow? You can probably get enough electricty to run some parts literally out of the air, as long as the various transmitters all around us keep running. And if they all stopped, you'd (we would all) likely have other bigger problems than some device failing.