Github user twalthr commented on the issue:
https://github.com/apache/flink/pull/6188
@fhueske `MONTH`, `QUARTER`, and `YEAR` are not converted into milliseconds
but treated as interval of months. In general, Flink supports two types of
intervals one for millis and one for months. So given that we support
`TIMESTAMP + INTERVAL_MONTHS` the behavior should be the same as a timestamp
add function.---
