Could be I was pretty wrong when thinking about blaming Nokia for that annoyance and was pretty right when stating it is imho a software problem.
When the N900 booted up once wihout t&d prompt, the only thing that was different from all the other battery-out-boots was: No 3rd party software, esp. no extras-testing,no extras-devel. Fresh and clean as a new born, no backups were applied etc.
So there comes the idea that some 3rd party app breaks t&d continuity. Could this possibly be a problem with paths a/o read/write permissions? I'll take a close look at log files the next days.
So: all of you happy owners of a user-friendly N900 that remembers time and date as it is supposed to: do you have rootsh and /or sudser installed? What other 3rd party software?
Can you find coincidence between the system's t&d 'behaviour' and the installation of a special appliction?