look in the css file of the custom skin for the style of <h2>

Here's what I have:

h2 {
	color: #333333;
	font-size: 180%;
	margin: 0;

and then:

* Styles for main area (left) */
h2 {
	margin: 1ex;
	border-bottom: 1px solid #333333;
}

I've tried omitting the border-bottom part but it's still showing the double line.

since I knew the default layout for the date was
date_if_changed( $before='<h2>', $after='</h2>', $format='' )
I thought is was that.
Can you give an URL, so I can look what class it is the date is shown in (or you can have a look.
look in your browser, do view-source and look for the date.
You then see the class, and you can then do a lookup for that class..

Sure, it's

www.bisu.nu/newsblog

apparently the view source just yields

<h2>06/22/05</h2>

Well, it is very clear that it is the this part in your CSS

* Styles for main area (left) */ 
h2 { 
   margin: 1ex; 
   border-bottom: 1px solid #333333; 
} 

if you replace it with :

* Styles for main area (left) */ 
h2 { 
   margin: 1ex; 
   border: 0px; 
} 

does that still shows up ?

Ah there we go, works perfectly. At first I tried just omitting the border-bottom line, instead of making it 0px. Thanks for the fix.