Scenario:
"I usually post more than one item on my blog each day. I want items of another day to be divided by some line with the date."
Solution:
Go to your Nucleus admin area, click on "Templates", edit the template you are using and go to the section "Date Header". Put something like this in it:
<div> <h6>%d %B</h6> </div>
Original forum thread (thanks, Armon!):
http://forum.nucleuscms.org/viewtopic.php?t=10940