It started happening to me recently, and no, it's not a problem that applies to all N900s, since obviously, there's an internal (crappy) battery in the N900.

what if that internal battery retired? 0.o can we replace it like bios battery?

I guess it can be replaced, kerio on IRC mentioned it's the same as the one in the OpenMoko Freerunner (joerg_rw should know the make in that), it's directly above the vibration motor, feel free to disassemble your N900, it's an easy process :P

Let the warranty pass a bit and time for a major overhaul
reserving those warranties in case of usb port malfunction

note that if you change the sim card it asks for time and date, but it is correct, you just have to press ok
________
Web shows

well, i dont know about you guys, but mine always retrieve the time with no problems, all i have to do is set the timezone.

Wonder if we could find a place to paste in the last timestamp
that the device was used so that at least when it starts up again
at least the year and month, and perhaps the day, are the same?
I have been wondering about that - I have some NAS drives
that reboot without a BIOS battery into the year 2001 or so,
it always seems such a pain to find the SYSLOG reports
going from 2010 back into prehistory.
All of our wifi units reboot from time to time and their syslogs
have ancient dates.
Having this issue on a handphone is an in-your-face bummer.

The business of losing date-time on a reboot can screw things up
where data that depends on well-ordered time-stamps are concerned.

This is not a problem specific to the n900,
but this issue suggests a solution is needed in linux itself.

Maybe at least the handphone manufacturers could insert
a small solar cell to keep the onboard battery company.

Just thinking out loud...

it used to happen to me but after countless re-flashes for other issues it stopped and have not had it since.

Yesterday I've remove the battery for about 2-3mins. I turned the phone on in 2 hours. There was no problem with the time and date. Have you tried with another battery?

Every of my Nokia phones has this problem after several month of massive usage. Some earlier. Some later... I guess this is a general failure by design.

My N900 began this behavior after 8 Months of usage. My N95 had this problem after about 20 Months...