> If I'm correct I must have a SP for my operation, or I can use these
> statements in regular SQL Statements?

You can use it directly

SELECT
EXTRACT(DAY FROM (MyDate - EXTRACT(DAY FROM MyDate) + 32
- EXTRACT(DAY FROM MyDate - EXTRACT(DAY FROM MyDate) + 32)))
FROM MyTable;

or store that expression into computed column.

Ivan