I have never got a good explanation to the dates in WH40K. Example: 3.305.778.M41

In this I understand that the year is 778 in Millenium 41. But waht does the 3.305 mean? 3rd hour in day 305?

Funny you asked, since this very topic was just posted about on a fansite not to long ago by Assandra.

http://darkreign40k.com/forum/index.php?topic=1548.0

Again the link thingie is being wierd for me, so just cut and paste the above link and it'll take you where you need to go.

You can also see exactly how the Imperial Calendar works here: www.scholaprogenium.com/calendar.html