Fix for LOGBACK-894 cleanByPeriodOffset add time check

Hi @tony19  <a href="https://github.com/qos-ch/logback/pull/219">219</a> 
```
    <p>Fixed <a href="http://jira.qos.ch/browse/LOGBACK-894">LOGBACK-894</a>, 
    <code>cleanByPeriodOffset</code> add time check.
    </p>
```