Elapsed filter error

Hi everybody, I'm trying to use the elapsed filter but I think it has a bug

this is my -cleaned- result

"@timestamp": "2016-09-20T01:52:21.000Z",
"tags": [
"elapsed_time": 128143.655,
"elapsed_timestamp_start": "2016-09-20T01:51:18.000Z"

as you can see between the two timestamps only 66s passed
instead the result is 128143 which are actually the seconds between the start_event timestamp and the submission of the end_event

both the events were submitted to logstash at 21 Sep 2016 13:27:01 GMT in a single block

It seems to me that it is using the timestamp of the start_event (correct) but the submission time of the end_event (wrong) I expect it to use the timestamp in both of them. Why is it having such a strange behavior?

my fault

i was actually applying elapsed filter after changing the timestamp of the event with the one I parse