Trim from only one field

Elastic Stack Logstash
1

Hello Loggers,

I am wondering how I could be able to trim the value of a string up to a certain point:
Example:
2016-03-19-06.00.00.000000

trim only:
2016-03-19-06.00.00.000

In my mappings the format is:
"Timestamp": {
"format": "yyyy-MM-dd-HH.mm.sss",
"type": "date"
}

Basically I want to remove the last 3 characters in the field.

Thanks a lot in advance guys,