Date difference scripted field in Kibana - elasticsearch

Question

I wanted to find the difference between two fields using scripted fields. Here are the two date fields and their format:
start_time - June 17th 2018, 09:44:46.000
end_time - June 17th 2018, 09:44:49.000
Which will give proc_time.
Here's what I am trying to do in scripted fields:
doc['start_time'].date.millis - doc['end_time'].date.millis
But this is returning the processing time which is deducted from epoch time.
For example, if my processing time is 2 seconds, then the output will be epoch time - 2 seconds.
Which is not what I want.
This is the sample doc:
17 Jun 2018 04:14:46 INFO CSG event file generation started at: Sun Jun 17 04:14:46 CDT 2018
17 Jun 2018 04:14:46 INFO Executing CSG file generation process
Warning: Using a password on the command line interface can be insecure.
17 Jun 2018 04:15:57 INFO Finished at: Sun Jun 17 04:15:57 CDT 2018
Any help would be appreciated.
Update
I've got this working with the following painless script:
((doc['csg_proc_end_time'].date.year) * 31536000 + doc['csg_proc_end_time'].date.monthOfYear * 86400 + doc['csg_proc_end_time'].date.dayOfMonth * 3600 + doc['csg_proc_end_time'].date.secondOfDay) - ((doc['csg_proc_start_time'].date.year) * 31536000 + doc['csg_proc_start_time'].date.monthOfYear * 86400 + doc['csg_proc_start_time'].date.dayOfMonth * 3600 + doc['csg_proc_start_time'].date.secondOfDay)
However, I would welcome any other script which does this in a simpler way.
JSON format for added fields:
"fields": {
"#timestamp": [
"2018-06-20T04:45:00.258Z"
],
"zimbra_proc_time": [
0
],
"csg_proc_time": [
71
],
"perftech_proc_time": [
0
],
"csg_proc_end_time": [
"2018-06-17T04:15:57.000Z"
],
"csg_proc_start_time": [
"2018-06-17T04:14:46.000Z"
]
},

Answer 1

This is what I've done to reproduce your issue and it works properly:
PUT test/doc/1
{
"csg_proc_end_time": "2018-06-17T04:15:57.000Z",
"csg_proc_start_time": "2018-06-17T04:14:46.000Z"
}
Now compute the processing time in a script field:
GET test/_search
{
"script_fields": {
"proc_time": {
"script": {
"source": "(doc.csg_proc_end_time.value.millis - doc.csg_proc_start_time.value.millis) / 1000"
}
}
}
}
Result: 71 seconds
{
"_index": "test",
"_type": "doc",
"_id": "1",
"_score": 1,
"fields": {
"proc_time": [
71
]
}
}