Missing default-operator-class at a "TIMESTAMP UNIQUE" column

System Configuration
---------------------
  Architecture (example: Intel Pentium)  	:  i686

  Operating System (example: Linux 2.0.26 ELF) 	:  linux 2.2.13 ELF

  PostgreSQL version (example: PostgreSQL-6.5.2):  PostgreSQL-6.5.2 

  Compiler used (example:  gcc 2.8.0)		:  egcs-2.91.66 19990314/Linux


Please enter a FULL description of your problem:
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Creating a column of type TIMESTAMP being constrained to be UNIQUE
results in the error

"ERROR:  Can't find a default operator class for type 1296."


Other date/time-types will be accepted.



Please describe a way to repeat the problem.   Please try to provide a
concise reproducible example, if at all possible: 
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test=> CREATE TABLE foo ( bar TIMESTAMP UNIQUE );
NOTICE:  CREATE TABLE/UNIQUE will create implicit index 'foo_bar_key' for table 'foo'
ERROR:  Can't find a default operator class for type 1296.