Sorting by date

We have a RSS feed with the following xml:

<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0"
 xmlns:aaa=".../">
  <channel>
    <title>Title1</title>
    <link>link1</link>
    <description>Description 1</description>
    <language>en-us</language>
    <item>
      <title>item 1</title>
      <description>item desc 1</description>
      <SortDate>Wed, 18 Feb 2008 11:18:41
-0000</SortDate>
      <link>item link1</link>
      <guid>item guid1 </guid>
      <aaa:modelDate>Sat, 15 Mar 2008 12:18:41
-0000</aaa:modelDate>
    </item>
    
    <item>
      <title>item 2</title>
      <description>item desc 2</description>
      <SortDate>Wed, 19 Feb 2008 11:18:41
-0000</SortDate>
      <link>item link1</link>
      <guid>item guid1 </guid>
      <aaa:modelDate>Sat, 16 Mar 2008 12:18:41
-0000</aaa:modelDate>
    </item>    
  </channel>
</rss>

I need to sort by date(SortDate) and prepare a html
file which outputs the following:

SortDate (yyyy-mm-dd format)
title: modelDate

Eg Output:
2008-02-19 ==> notice that Feb 19 item is displayed
1st
item 2

2008-02-18
item 1

How do I sort the out by date?