Timestamps are interpreted to be timezoneless and stored as an offset from the UNIX epoch. Convenience UDFs for conversion to and from timezones are provided (to_utc_timestamp, from_utc_timestamp).
All existing datetime UDFs (month, day, year, hour, etc.) work with the TIMESTAMP data type.
The Decimal datatype in Hive based on Java's BigDecimal which is used for representing immutable arbitrary precision decimal numbers in Java.
With the present implementation, the Decimal datatype has unlimited precision. While this has absolutely no impact on usability, it's worth noting for performance reasons. HIVE-3796 is tracking the progress of allowing users to specify scale and precision when creating tables with decimal datatype.
Union types can at any one point hold exactly one of their specified data types. You can create an instance of this type using the