How to correctly parse and index dates into timestamp that have no offset? (Post title renamed)

Elastic Stack Logstash
3

Thanks for the pointer. Before reading your reply I tried extending my mapping to support the date format.
<<
properties": {
"@timestamp": {
"type": "date",
"format": "YYYY/MM/dd HH:mm:ss||strict_date_optional_time||epoch_millis"

I also added the following the timezone thinking it might help

<<
date {
match => [ "err_time" , 'YYYY/MM/dd HH:mm:ss' ]
locale => en
timezone => "UTC"
}

Should I use add_field for the offset being one is not included in the source logfile? Are you able to provide example syntax for inserting the offset for dates that do not contain them?

Many thanks in advance.

Mark.