OK, i have done some more testing and in order to accomplish this we need to use the php timestamp to be able to subtract the 30 days. first, please update your date column to use the timestamp. This will store the date in the following format:
Then your calculation should be something similar to this:
strtotime([dateColumn1]) - (30*24*60*60)
basically we convert the date to a timestamp and then subtract 30 days.
let me know if you have any questions.