Why are dates saved as numbers ?

(from github.com/attibalazs)
I noticed that some dates are saved as numbers. I want to use Kibana to look at the indexes and do some analytics on them but the dates show up as numbers. I am looking at the .crawler index where dates are stored in the epoch format as numbers.

I noticed you are displaying the dates correctly in FESS, is this related to elastic search or the way fess saves the data ?

“status”: 9999,
“httpStatusCode”: 200,
“method”: “GET”,
“mimeType”: “text/plain”,
"createTime": 1471282801243,
“executionTime”: 23,
“contentLength”: 219,
"lastModified": 1340626665470

(from github.com/marevol)
It’s a specification for Fess.
We do not expect data in .crawler index is displayed on Kibana.