You're asking for two things:
(1) Get date information out of that string. Because it's an ambiguous format, you'll have to do it manually: there are a number of threads on this subject: search for parse date string to find some.
(2) Put that date information into a PostgreSQL query: how to do this depends on PostgreSQL, the preferred format for which is the ISO standard 'yyyy-mm-dd'. Put the year, month, and day into a string like that, put that string into the SELECT query.