title: log4j2 RolllingFileAppender filePattern create a wrong date
body: <p>I used date in filePattern, but the date log4j create is not right.
  Today is 2015-11-23, but log4j create a file directory named 2015-12.
  my log4j2 version is 2.4.1.
  <a href="http://i.stack.imgur.com/MCImI.png" rel="nofollow">here is a snapshot</a></p>
  
  <p>below is my log4j2 configuration.</p>
  
  <pre><code>&lt;RollingFile name="RollingFile" fileName="logs/executor.log" append="true"
                   filePattern="logs/$${date:yyyy-MM}/executor-%d{yyyy--MM--dd}-%i.log.gz"&gt;
        &lt;PatternLayout&gt;
          &lt;Pattern&gt;%t  %d{yyyy-MM-dd HH:mm:ss} %c %p -%m%n&lt;/Pattern&gt;
        &lt;/PatternLayout&gt;
        &lt;Policies&gt;
          &lt;TimeBasedTriggeringPolicy interval="24" modulate="true"/&gt;
          &lt;SizeBasedTriggeringPolicy size="1MB"/&gt;
        &lt;/Policies&gt;
      &lt;/RollingFile&gt;
  </code></pre>