It is pretty easy. First of all, move the mouse over the date, you will see a key that will happear on the side. If you click on it, you will see all the available options for the date line.
In the "universal" tab you can put the date and time display format. (In my screenshot, my config is: %A, %d %B %Y - %H:%M)
Once you do this click set, then close.
If you do not display the config you have choosen, start looking inside the panels you find in Omnimo 4. In the WP7/TextItems/Day you will find a variant called "Universal.ini". Load that and you'll have the date and time on screen like you want.
I tried to be as clear as I could, since english is not my main language. If you encounted further problems, you can contact =fediaFedia He is the creator of Omnimo 4.