The date is a couple of span's and some CSS. This is the generated source:
<p class="post-date">
<span class="month">sep</span>
<span class="day">1</span>
</p>
The CSS for the month part:
.post .post-date span.month {
-moz-background-clip:border;
-moz-background-inline-policy:continuous;
-moz-background-origin:padding;
background:#F44365 none repeat scroll 0 0;
border-bottom:1px solid #FFFFFF;
color:#FFFFFF;
font-size:85%;
left:0;
padding:2px 0;
position:absolute;
text-transform:uppercase;
top:0;
width:100%;
}
And the day part:
.post .post-date span.day {
bottom:1px;
left:0;
padding:3px 0;
position:absolute;
width:100%;
}
I don't know how your theme generates its date but there is good chance it used this function. You'll have to edit that to get the markup you need.