SELECT timestamp('2001-06-12'::date - '2000-06-12'::date)

The "+" operator is not defined for (date, date) which seems very
logical to me since adding dates does in my opinion not make sense.
performance=# SELECT timestamp('2001-06-12'::date + '2000-06-12'::date);

ERROR: Unable to identify an operator '+' for types 'date' and 'date'
You will have to retype this query using an explicit cast

Surprisingly "-" is defined for (date, date).
performance=# SELECT timestamp('2001-06-12'::date - '2000-06-12'::date);

timestamp
------------------------
1970-01-01 01:06:05+01
(1 row)

Does anybody understand the logic behind that operation and why is the
result not something like 1970-01-01 00:00:00?

Hans