Using an SQL Date Formula in OBIEE Report

Summary

How do I display previous 30 days of data

Content

Could someone please show me how to tweak the below formula.  I’d like to pull data from the previous 30 days; however, it seems to be pulling for the future 30 days instead. 

               timestampadd(sql_tsi_day, 30, current_date)

When I change the formula to the below example it gives me an error and doesn't return any data:

                timestampadd(sql_tsi_day, -(30), current_date)

Thank you.

Version

OBIEE 11.1.1.9.0