I apologize, I should have stated I was using the JSON codec with the Kinesis input for clarity.
As it turns out, I don't need either the date filter or the mutate to make it work. I inherited the configuration and just assumed that it was working all along. I had never checked the output and thus did not see the _dateparsefailure messages until now.
Thank you, and thanks for the link to the issue. I searched Logstash issues but not the specific plugin repository issues; now I know better.