SQL Plus Query question

I am doing a data scrub query and I have most of it done except the following:
I need to exclude all patients that have had a first shipment one month prior to the date this report will run.

Please help!

WHERE...

patients.first_ship_date >= ADD_MONTHS(sysdate, -1)

obv, left col has to be your own table and column, and the col must be an Oracle DATE data type. If it's a string containing some date representation, then wrap it with Oracle's TO_DATE(table.the_column, 'mm/dd/yyorotherformatspecifier')