When one of our Dash 1.2e boards running the latest 0.10.3 firmware was connected to a new battery after draining the old battery below its undervoltage cutoff, the RTC appears to have stopped ticking. That is, Clock.counter() returned the same value, which had been synchronized with network time, in subsequent calls an hour later. Dash was in deep sleep and the modem/system was powered down. After about 2.5 hours the clock started working again, without any reset.
I haven’t come across this issue before, so I thought I’d mention it FYI.