Watcher transform runtime error illegal_argument_exception: "dynamic method [java.lang.String, getMillis/0] not found"

Hi, I've created a watch containing transform for which I get a runtime error when executing:

        "type" : "illegal_argument_exception",
        "reason" : "dynamic method [java.lang.String, getMillis/0] not found"
       "script_stack" : [
        "tmpdate = Instant.ofEpochMilli(ctx.payload.hits.hits[j]._source.eventTime.getMillis()).atZone(ZoneId.of('Europe/Zagreb')).format(DateTimeFormatter.ofPattern('YYYY-MM-dd HH:mm:ss')); ",
        "                                                                         ^---- HERE"
      ],

The whole transform is here (I've break the lines for easier readibility):

  "transform":{
      "script": {
		"suorce": "
			ctx.payload.transform = []; 
			def document = []; 
			for (int j=0;j<ctx.payload.hits.hits;j++)
			{
				def tmpdate = Instant.ofEpochMilli(ctx.payload.hits.hits[j]._source.eventTime.getMillis()).atZone(ZoneId.of('Europe/Zagreb')).format(DateTimeFormatter.ofPattern('YYYY-MM-dd HH:mm:ss'));; 
				document = [
					'amount': ctx.payload.hits.hits[j]._source.amount, 
					'eventTime': tmpdate
				]; 
				ctx.payload.transform.add(document) 
			} 
			return ['totalCnt': ctx.payload.hits.total, 'transformedHits': ctx.payload.transform];",
          "lang": "painless"
      }
  }

Any help to get me on the right line is appreciated.

EDIT:
Maping for field eventTime is:

"eventTime": { "type": "date" }

EDIT2:
Watcher executes and returns this document inside hits:

      "hits" : {
        "hits" : [
          {
            "_index" : "my_requests",
            "_type" : "_doc",
            "_source" : {
              "amount" : 1200.0,
              "eventTime" : "2019-07-23T12:40:26.9739999Z",
              "@version" : "1",
              "status" : "0"
            },
            "_id" : "bb64b6af3c374cfb9ae2ec107ec2310d",
            "sort" : [
              1563885626973
            ],
            "_score" : null
          }, ...

OK, to answer my own question, this error is obviously stating that I'm trying to do a datetime function on a string object (even my field is mapped as date ???) so I've done the following in order to get the date field from UTC to local time:

def tmpdate = ZonedDateTime.parse(ctx.payload.hits.hits[j]._source.eventTime).withZoneSameInstant(ZoneId.of('Europe/Zagreb')).format(DateTimeFormatter.ofPattern('YYYY-MM-dd HH:mm:ss'));

accessing the source will only return JSON data structures like string/bool/integer, but not parse to a date. This only happens when doc values are used (i.e. for sorting or for script fields).

hope this helps!