Found this thread as a result of having the same problem ever since I bought my spare battery a few weeks ago (I guess we are all getting to that stage now with this device - roll on the N9)

Aaaaanyway, I followed an earlier posters advice and swapped them really quickly - I mean less than a second to swap over - and hey presto - memory of date/time is retained.

That'll do for me. Thanks.

My N900 got the time when I used a network with NITZ, now I am with t-mobile.de and I wish that I could disable that screen that asks me about date and time every time I swap batteries. I have enabled ntpd, but that takes some time to kick in.

There is OpenNTPD
http://en.wikipedia.org/wiki/OpenNTPD

OpenNTPD is a Unix system daemon implementing the Network Time Protocol to synchronize the local clock of a computer system with remote NTP servers.

http://wiki.maemo.org/NTP

Nokia-N900:~# apt-cache search ntpd
openntpd - OpenBSD NTP daemon

Nokia-N900:~# apt-get install openntpd
The following NEW packages will be installed:
openntpd
Need to get 47.0kB of archives.
Setting up openntpd (3.9p1-4-maemo8) ...
Starting openntpd: ntpd.

Configuration
Open file /etc/openntpd/ntpd.conf with your favourite text editor and simply put this line into it:

servers europe.pool.ntp.org

Or any other near server from http://www.pool.ntp.org/

After that, save the file and reboot the device. Everything is working now in the background and you will always have correct time set. Don't worry, the daemon does not drain battery.

And keep in mind that you have to wait some time for the effect to be seen, because ntpd calculates drift first and does not do rapid changes (to keep consistent log files). You can test if it works if you set your time a few minutes back or forth and leave the device connected. The results after initial installation should be seen in an hour or two, after that set your other clocks by your phone's time.