Aggs Painless DateTime Modify inline

[AndyS (Andrew Shepherd)]

Hia,
As per title I'm stuck with a painless timezone Q:

Customer, WebUI, tz UTC+5
The WebUI issues a query and converts query to UTC
Elastic processes in UTC and then aggs by hour and by minute

"aggs": {
	"time": {
		"terms": {
			"script": {
				"source": "doc['@timestamp'].value.getDayOfMonth()"
			}
		},
		"aggs": {
			"time": {
				"terms": {
					"script": {
						"source": "doc['@timestamp'].value.getHour()"
					}
				}
			}
		}
	}
},

The results accurately group into buckets

  "aggregations" : {
    "time" : {
      "doc_count_error_upper_bound" : 0,
      "sum_other_doc_count" : 0,
      "buckets" : [
        {
          "key" : "10",
          "doc_count" : 484318,
          "time" : {
            "doc_count_error_upper_bound" : 0,
            "sum_other_doc_count" : 0,
            "buckets" : [
              {
                "key" : "17",

But the reply to the WebUI is now in UTC. Converting from key:10 and key:17 is possible in JS, but fiddly (as it could be getDay() + getHour().... or getHour() + getMinute().

I suspect the cleanest way to do this is a tz modifier before the getHour() is issued. I've spent a few hours looking at Java/Painless but I'm struggling, every example I can find is fundamentally different, or I can't work out the alternative.

I guess the code would look something like:
"source": "doc['@timestamp'].setTimeZone('America/Argentina/Buenos_Aires').value.getHour()"

Any input much appreciated!

Thanks
Andy