Add a new field with a calculation on every document insertion

My document looks like

{ "start_time" : "2016-Sep-04 18:06:24.352", "end_time" : "2016-Sep-04 18:06:28.352" }

When I inserted this document into the ES I want it to become as

{ "start_time" : "2016-Sep-04 18:06:24.352", "end_time" : "2016-Sep-04 18:06:28.352", "difference": 2000 }

The difference is in millisecond. Is there any way top acheive this. I seen scripting, updatebyquery & so on. But I dont get any idea. Please help me with this????

Is there anyway for this?????

For now the only way to do it is on client side. In the next version of elasticsearch (5.0) we are adding capabilities in order to enrich the source, see