Cache SQL Query-- "In the last XX days"

Intersystems Cache SQL Query needed to determine all true results in the last 30 days. DATEDIFF isn't the correct way to go-- I get results form that specific date in the past, not SINCE that date-- and the interface is hostile to using "[]" square brackets in the query. I'm stuck. Luckily I have a field called "Current_date" to use so I needn't use system time.