I have tested a date filter that imports the metadata.event_timestamp correctly when used in a custom parser. I set the timezone in the date filter and the tz offset is applied to the time so that the metadata.event_timestamp is correct in Z time.
When I test the same date filter when creating a parser extension, the UDM output preview of the metadata.event_timestamp has the original time without the tz offset applied.
Can a parser extension be used to overwrite event_timestamp?
The parser extension will overwrite a value set by the default or custom parser. The exception to this is with a repeated field. You can configure the parser extension to append data when writing to repeated fields. Hopefully that helps?