Apache Beam: why is the timestamp of aggregate value in Global Window 9223371950454775?

We migrated from Google Dataflow 1.9 to Apache Beam 0.6. We are noticing a change in the behavior to the timestamps after applying the globalwindow. In Google Dataflow 1.9, we would get the correct timestamps in the DoFn after windowing/combine function. Now we get some huge value for the timestamp e.g. 9223371950454775, Did the default behavior for the globalwindow change in Apache Beam version?

input.apply(name(id, "Assign To Shard"), ParDo.of(new AssignToTest()))
      .apply(name(id, "Window"), Window
          .<KV<Long, ObjectNode >>into(new GlobalWindows())
          .triggering(Repeatedly.forever(
              AfterProcessingTime
                  .pastFirstElementInPane()
                  .plusDelayOf(Duration.standardMinutes(1))))
          .discardingFiredPanes())
      .apply(name(id, "Group By Shard"), GroupByKey.create())
      .appy(.....) }