EventLogging data as it has been most commonly used in recent years, i.e. in the form of MySQL/ MariaDB tables, contains a human-readable timestamp field in the MediaWiki format ( yyyymmddhhmmss ). As discussed in T179540: Timestamp format in Hive-refined EventLogging tables is incompatible with MySQL version, the new Hive-refined EL data ended up using epoch timestamps instead, but for reasons of downward compatibility and to facilitate joins, we still need to be able to use MediaWiki timestamps too.
In T179540#3742635, @Ottomata proposed that the Analytics Engineering team could provide a UDF for this, "something along the lines of SELECT MediawikiTimestamp(dt) ...."