Hive currently uses the "local" time of a java.sql.Timestamp to represent the SQL data type TIMESTAMP WITHOUT TIME ZONE. The purpose is to be able to use Timestamp#getYear() and similar methods to implement SQL functions like year.
When the SQL session's time zone is a DST zone, such as America/Los_Angeles that alternates between PST and PDT, there are times that cannot be represented because the effective zone skips them.
Using UTC instead of the SQL session time zone as the underlying zone for a java.sql.Timestamp avoids this bug, while still returning correct values for getYear etc. Using UTC as the convenience representation (timestamp without time zone has no real zone) would make timestamp calculations more consistent and avoid similar problems in the future.
Notably, this would break the unix_timestamp UDF that specifies the result is with respect to "the default timezone and default locale". That function would need to be updated to use the System.getProperty("user.timezone") zone.