XSLT 1.0 Substring then select distinct

I am quite new to xslt, so any help will be much appreciated. Below is my sample xml file.

 <DocumentElement>
     <Records>
       <date>2014-07-01 00:00</date>
    </Records>

    <Records>
       <date>2014-08-03 00:00</date>
    </Records>
     <Records>
       <date>2013-08-03 00:00</date>
    </Records>
 <DocumentElement>

What I need is just to select distinct years from the dates.

Currently I have the below xslt which brings duplicate years.

<?xml version="1.0" encoding="utf-8"?>
 <xsl:stylesheet version="1.0" xmlns:xsl="http://ift.tt/tCZ8VR"
xmlns:msxsl="urn:schemas-microsoft-com:xslt" exclude-result-prefixes="msxsl" xmlns:ms="urn:schemas-microsoft-com:xslt">
 <xsl:output method="xml" indent="yes"/>

  <xsl:template match="/">

<xsl:variable name="years" select="DocumentElement/Records/date"/>

<ul>
  <xsl:for-each select="$years">
    <li>
      <xsl:element name="a">

        <xsl:attribute name="href">
          <xsl:value-of select="concat('?archive=',substring( ., 1, 4))"/>
        </xsl:attribute>

        <xsl:value-of select="substring( ., 1, 4)"/>
      </xsl:element>

    </li>
  </xsl:for-each>
</ul>
  </xsl:template>

 </xsl:stylesheet>

Which gives me the results below:

<ul>
  <li>
    <a href="?archive=2014">2014</a>
 </li>
 <li>
  <a href="?archive=2014">2014</a>
 </li>
 <li>
   <a href="?archive=2014">2013</a>
  </li>
 </ul>

But my expected result should be

  <ul>
  <li>
    <a href="?archive=2014">2014</a>
 </li>

 <li>
   <a href="?archive=2014">2013</a>
  </li>
 </ul>

I tried the following below but I get empty output

   <?xml version="1.0" encoding="utf-8"?>
      <xsl:stylesheet version="1.0" xmlns:xsl="http://ift.tt/tCZ8VR"
   xmlns:msxsl="urn:schemas-microsoft-com:xslt" exclude-result-prefixes="msxsl" xmlns:ms="urn:schemas-microsoft-com:xslt">
  <xsl:output method="xml" indent="yes"/>


  <xsl:template match="/">

   <xsl:variable name="years" select="substring(DocumentElement/Records/date, 1, 4)"/>

   <ul>
  <xsl:for-each select="$years[not(.=preceding::*)]">
    <li>
      <xsl:element name="a">

        <xsl:attribute name="href">
          <xsl:value-of select="concat('?archive=',substring( ., 1, 4))"/>
        </xsl:attribute>

        <xsl:value-of select="substring( ., 1, 4)"/>
      </xsl:element>

    </li>
  </xsl:for-each>
</ul>
</xsl:template>

   </xsl:stylesheet>

Any help will be much appreciated. thanks.