And don't forget to vote on these related bugs:

Clock should allow configurable 12h/24h display
Date format should be user defined

Nokia have hard coded date and time formats for specific regional locations - so UK Brits (Region: English (United Kingdom)) get a 12-hour clock and dd/mm/yyyy formats, because in Nokias opinion everyone in Great Britain uses a 12-hour clock... it would be funny if it wasn't so stupid and annoying - I live in Britain and prefer to use a 24-hour clock to avoid ambiguity.

One possible solution is to tell the N800 your region is Nederlands or Dutch - the trouble then is that the decimal and thousands seperators are transposed, and you might not like the date format!

No sign of this being fixed any time soon.