How can I calculate this?

Select
time '15:00:0.000' - time '14:04:42.000',
cast('15:00:0.000' as time) - Cast('14:04:42.000' as time),
cast(3318.0000 as time),
cast(cast('15:00:0.000' as time) - Cast('14:04:42.000' as time) as time)
From rdb$Database;

The first two even work, but I can not transfom them into time fields.



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