how do you transform a date that's storred as a type long (epoch time) into a dateOptionalTime in Elasticsearch? - elasticsearch

Question

I have a field in my database that's stored as Epoch time, which is a long. I'm trying to get Elasticsearch to recognize this field as what it actually is: a date. Once indexed by Elasticsearch, I want it to be of type dateOptionalTime.
My thinking is that I need to apply a transform to convert the Epoch long into a string date.
On my index, I have a mapping that specifies the type for my field as date with a format of dateOptionalTime. Finally, this timestamp is in all of my docs, so I've added my (attempted) mapping to _default_.
The code:
'_default_', {
'_all': {'enabled': 'true'},
'dynamic_templates': [
{ "date_fixer" : {
"match": "my_timestamp",
"mapping": {
"transform": {
"script": "ctx._source['my_timestamp'] = new Date(ctx._source['my_timestamp']).toString()"
},
'type': "date",
}
}
}]
}
I'm brand new to Elastic, so I'll walk through what I think is happening.
I'm setting this to type _default_ which will apply this to all new types Elastic encounters.
I set _all to enabled. I want Elastic to use the default mapping for all types with the exception of my timestamp field.
finally, I add my dynamic template that (a) converts the long into a date, and (b) applies a mapping to the timestamp field explicitly saying that it is a date
The Problem:
When I try to add my data to the index, I get the following exception.
TransportError(400, u'MapperParsingException[failed to parse [my_timestamp]]; nested: ElasticsearchIllegalArgumentException[unknown property [$date]]; ')
My data looks like:
{
"timestamp": 8374747594,
"owner": "text",
"some_more": {
"key": "val",
"key": "val"
}
}