Github user fhueske commented on the issue:
https://github.com/apache/flink/pull/6188
Hmm, I think there is still a case for `timestampAdd`. The problem are time
intervals with variable length such as `MONTH`, `QUARTER`, and `YEAR`. All of
these cannot be defined by milliseconds, because they depend on the context. So
`timstampAdd(ts, 1.month)` returns a different result than `ts + 1.month`.---
