title: Flink Timestamp monotony violated
body: <p>I'm consuming data using a Kafka Consumer 0.9 and after some initial filtering for valid events and mapping to POJO I am setting AscendingTimestampExtractor.</p>
  
  <p>using Joda Time library I'm returning the milliseconds since epoch as follows:
  <code>return DateTime.parse(evnt.timestamp).getMillis();</code></p>
  
  <p>This seems to result in 
  <code>WARN  org.apache.flink.streaming.api.functions.timestamps.AscendingTimestampExtractor  - Timestamp monotony violated: 1478048406982 &lt; 1478051502295</code></p>
  
  <p>Of course the ordering is different but what I don't understand is how make sure these are in order? Should I be ordering these based on this timestamp and then performing any keyBy or timeWindow?</p>
  
  <p>I haven't really found much information on this so any help would be appreciated.</p>