I found something similar on other freescale socs. If date is too old (say 2005) the rtc won't tick. You won't find this behaviour often because the hwclock will drain a bit of battery to keep the time in sync, but if the battery is really empty this can happen.
I'm not sure if Pocketbook has a menu to set date & time from its gui. For Cervantes (which can run with KOReader only) a workaround is ->
https://github.com/koreader/koreader...one.sh#L10-L15