NTP
NTP stands for a Network Time Protocol and it is used for keeping device's clock 100% correct by connecting to the NTP servers when needed and getting the accurate time. There is a similar feature in N900's Settings -> Date and time -> Update automatically, but it is for updating the time via mobile network. Keeping the time correct with NTP is better solution, because it is not dependent on mobile network, more common and standard, and you have control over it.
[edit] Installation
There is a NTP client in extras, OpenNTPD. It is a widespread Unix/Linux/BSD daemon, which calculates your clock's drift and therefore connects when it is needed, not by schedule. So, enable extras if necessary (it's enabled by default on the N900) and install the OpenNTPD package.
[edit] Configuration
Open file /etc/openntpd/ntpd.conf with your favourite text editor and simply put this line into it:
servers europe.pool.ntp.org
The example configuration file uses European NTP servers. The list by continents and countries is on ntp.org. The servers used in this configuration are from pool.ntp.org. Those servers are one of the most accurate in the world. They relay the correct time, which is obtained from institutes, governments, GPS, etc. therefore from atomic clocks.
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.