The GUI does not provide a way to change the way date/time is represented on the n770. I prefer 24h clocks and DD-MM-YYYY dates. This is a quick hack to enable that on this device.
echo 'export LC_TIME=da_DK' >> /etc/osso-af-init/locale
That's it! It just uses the Danish locale for dates, which happens to be my preferred format. One quirk is that occasionally text relating to dates/times will show up in Danish. In particular, this has come up with the Weather application. I consider this particular quirk to be an oppertunity to learn some words of a different language ;-)
This trick also works on most GNU/Linux systems where you can't easily set the system-wide date/time format. For most systems, you can do this instead:
echo 'LC_TIME=en_DK.UTF-8' >> /etc/environment
You may need to install/generate the locales for Debian or Ubuntu systems, which can be done with dpkg-reconfigure locales and sudo locale-gen en_DK.UTF-8 respectively.