Many users reported that it is not possible for now to define an event-time field or watermark on a BIGINT column.
Impala supports to convert BIGINT into TIMESTAMP value using TO_TIMESTAMP() function . The BIGINT argument represents the number of seconds past the epoch.
I think it makes sense to support this for Flink's TO_TIMESTAMP(). It is the converse of the UNIX_TIMESTAMP() function, which produces a BIGINT representing the number of seconds past the epoch.