(Elasticsearch) Convert Unix formatted data into timestamp (without changing the mappings) - php

Question

We're executing an Elasticsearch query like this using PHP API:
$params = [
//please ignore the variables below,
//we made it in dynamic parameter-based in our function,
//that's why they're variables
'index' => $ourIndex,
'type' => $ourType,
'from' => $from,
'size' => $page_size,
'body' => [
"query" => [
'bool' => [
'must' => [
[
"query_string" => [
"default_field" => $content,
"query" => "$keywords"
]
],
[
"range" => [
"#timestamp" => [
"from" => $parseParams['pub_date_start'],
"to" => $parseParams['pub_date_end'],
'format' => "yy-MMM-dd'T'HH:mm:ss.SSS'Z'",
]
]
]
]
]
]
]
];
The query above works with our #timestamp field because its type is on date
"#timestamp" : {
"type" : "date"
}
And a sample value is like this:
"#timestamp" : "2019-06-17T16:53:55.778Z"
However, we want to target our pub_date field in our index, and in its mapping, the field has a type of long
"pub_date" : {
"type" : "long"
},
so it has this kind of values when we're displaying the documents:
"pub_date" : 1510358400
When we changed the query above to target instead of #timestamp to pub_date, it now displays an error like this:
Tried Solutions
I tried to add an additional format epoch_millis in the format property:
[
"range" => [
"pub_date" => [
"from" => $parseParams['pub_date_start'],
"to" => $parseParams['pub_date_end'],
'format' => "yyyy-MM-dd||yy-MMM-dd'T'HH:mm:ss.SSS'Z'||epoch_millis",
]
]
]
But still fails
Main Question
I feel that the Unix formatted values cant be recognized by the range query of Elasticsearch so that's why the query fails. Is there a work-around for this without changing the MAPPINGS of the index?
Because the other possible solutions suggested to change the mapping, but we already have around 25 million documents in the index, so we thought formatting it in PHP would be a nicer approach

Answer 1

Since the field is of type long and stores the unix timestamp, simply convert the date in $parseParams['pub_date_start'] and $parseParams['pub_date_end'] to unix timestamp using strtotime. Update the range query as below:
"range" => [
"pub_date" => [
"from" => strtotime($parseParams['pub_date_start']),
"to" => strtotime($parseParams['pub_date_end']),
]
]