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

System Configuration
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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.