Create an archive listing based on meta array

Have you looked at PHP’s date function?

A quick example :

 $dates = $wpdb->get_col( /*your sql*/ );

 if( $dates )
 {
      $archive_year = "";

      foreach( $dates as $date )
      {
           $the_date = strtotime( $date );

           $year = date( 'Y', $the_date ); //4 digit year, ex 1999
           $month = date( 'F', $the_date ); //Full textual month, ex January

           if( $year != $archive_year  )
           {
                if( !empty( $archive_year ) ) echo "</ul>";

                echo "
                     <h2>$year</h2>
                     <ul>
                          <li>$month</li>
                ";

           }
           else
           {
                echo "<li>$month</li>";

           }

      }
 }

Second example

 $dates = $wpdb->get_col( /*your sql*/ );

 if( $dates )
 {
      $archive_year = date( 'Y' ); //Get current year

      echo "<h2>$archive_year</h2>";

      echo "<ul>";

      foreach( $dates as $date )
      {
           $the_date = strtotime( $date );

           $year = date( 'Y', $the_date ); //4 digit year, ex 1999
           $month = date( 'F', $the_date ); //Full textual month, ex January

           if( $year == $archive_year  )
           {


                echo "<li>$month</li>";

           }

      }

      echo "</ul>";

 }