Setting elastichsearch datasource timestamp

Hi all,
I’m using elasticsearch 6.4.2 as db and I want to define a grafana datasource on an index that we call ‘webwams’ .
Actually the json stored for my dynamic type of my index are like this:

/webwams/my_type/    

{
    "magnitude" : 0.22864656066894531,
    "phase" : 44.902667641471815,
    "time" : 1.53259831858E9,
	"time_ms" : 1.53259831858E9
 }

So when I try to refer to time_ms as time field name in definition of grafana datasource I got this:

anyone can help me with this issue?