How to convert existing timestamp format to another timestamp format using logstash?

yeah @magnusbaeck

But why does this even matter?

i want fields as a date type.

It would be great help if you can show an example snippet of code by using above scenario for converting fields date type.
and yeah,
These event matter for me because when i do same thing with both kind of logs then it have standard timestamp format at one place for both timestamp in consolidated form so i can visualize these thing easily in kibana dashboard and it would be helpful to understand end user.