Return type of TIMESTAMP_ADD applied to a DATE should be TIMESTAMP if unit is smaller than DAY

timestamp_add("MINUTE", 1, date '2016-06-15') returns 2016-06-15 since it returns a date and therefore truncates the minute informtion. timestamp_add should return timestamp instead of date for units less than date.

timestamp_diff with date doesn't handle null properly (in type inference), converted type is INTEGER NOT NULL (when it should be null).