Conditional Formatting as a countdown timer?

I have the following spreadsheet and i am looking to automate when people need training.

The idea is that when they enter the date they recieved their PDR it auto posts the date it needs re doing in column C. To emphasise it and make it easier i am trying to use the formula is part of conditional formatting so that the spreadsheet goes green when date entered, orange when there is 30 days till renewal and red when its over due.

I've been trying formulas such as =c2-today()<30 in the conditional formatting box but i cant seem to get it to work. Please could someone help?