My N900 _always_ lost time & date since I bought it (6 months, running FW 1.1, 1.2, 1.3).
Last week I had to flash rootfs [first time] cause my optification-trial script was buggy an messed up /usr/share
The day after I took out the battery to put in my SIM-card again and -WOW! - the device *did* remember the correct time & date!
Anyway, the next battery-outs brought back the old well-known annoying time & date prompt.
Since it happened only once and never again in months,
it imho cannot be a hardware-related issue [secondary battery broken/fixed/broken again ;].

Has anyone asked Nokia support /local NSC yet?
I'd be interested in any offical answer related to that annoying issue.

CB