Elasticsearch not able to identify the time field defined in Fluentd

Issue: I have the logs getting logged into my log file and each log entry has the time information. The time field in the log is not getting recognized in Elasticssearch and hence I am not able to select this time information while creating my Kibana indexes.

Ubuntu 18.04
Fluentd 1.12.3
td-agent 4 (curl -L https://toolbelt.treasuredata.com/sh/install-ubuntu-bionic-td-agent4.sh | sh)

@type tail
path /var/log/sau.log
pos_file /var/log/td-agent/sau.pos
read_from_head true
tag log
@type regexp
expression /^\[(?<logtime>[^\]]*)\] (?<name>[^ ]*) (?<title>[^ ]*) (?<id>\d*)$/
time_key logtime
keep_time_key true
time_format %Y-%m-%d %H:%M:%S %z
types id:integer
<match *.**>
@type elasticsearch
host localhost
port 9200
index_name sauindx1

log entry sample: [2013-02-28 12:00:00 +0900] sau engineer 1
log entry simulation command: echo “[2013-02-28 12:00:00 +0900] sau engineer 1” | tee -a /var/log/sau.log
Official doc referred: regexp - Fluentd

log entry in stdout through fluentd:
2013-02-28 03:00:00.000000000 +0000 log: {“logtime”:“2013-02-28 12:00:00 +0900”,“name”:“sau”,“title”:“engineer”,“id”:1}

As shown in the above line, when I do an stdout print from Fluentd, I get the above line which clearly indicates that the time field defined by me is recognized properly.
However when I divert the logs to Elasticsearch and then create the Kibana index, I am not able to get the time field recognized.

Is there anything I am doing wrong here? Needed some guidance on this please.

Reference links:
Elasticsearch installation – Install Elasticsearch with Debian Package | Elasticsearch Guide [7.12] | Elastic
Kibana installation – Install Kibana with Debian package | Kibana Guide [7.12] | Elastic
Fluentd installation – Install by DEB Package (Debian/Ubuntu) - Fluentd
Regex expression – regexp - Fluentd

I was able to figure it out with some help. I had to perform the conversion of datetime to the ISO8601 format which the elasticsearch understands. Did that using the below snippet:
<filter *.**>
@type record_transformer
auto_typecast true

datetime_received ${require ‘time’; Time.parse(record[“logtime”].to_s + " +0530").iso8601.to_s}

Regards, Saurabh