Set alert 30 days prior to date

I have a spread sheet and in column C and D i have dates in the cells in each column. I need an alert to show 30 days prior to the date in each cell in each column. I tried yahoo an this is what i got back and it did not work...

(Select your column
go to the Home Ribbon and then click on "Conditional Formatting"
Choose "New Rule"
Choose "format only cells that contain"
In the second dropdown near the bottom, select "less than"
Enter =TODAY()-30 in the field to the right.
Click on Format > Fill and chose your color (red)
Click OK then OK again.)


I would much appreciate any help. Thank you...