A: Wouldn't have a clue!

That's about it, anyway. "Of late" the phone connected to the NBN has been
losing time/resetting to some standard time. It's a Telstra model 12850, with
base station and (now, after one lost) two walkabout handsets.

No matter HOW MANY times I fix it up, it soon goes back to... - somewhere in 2011

At first I thought it was the batteries, but even though they're old, they stay fully
charged and seem to work OK. I remember (pre-NBN), even after a flat battery, I have
never had to re-set the time.

After it re-sets to the 2011 time, it then seems to "keep time" OK

Has anyone else had this problem? Do you know what causes it?