How to use conditional formatting 30 day notice

I have an excel document that I have set up some conditional formatting and use of icon sets. The only issue I am having, is programming the 30 day alert notice using conditional formatting. Here is my example of what I have in my excel document.

Cell (C3 holds the certification date) for example (Certified on 9/20/2017) is the certification date. Now I am using icon sets in cell C3 to either show me a green check mark if certification is valid and a red check mark if expired. My goal is to use another conditional format to highlight the C3 cell (yellow) when the certification date is approaching the 30 day expiration date. Does anyone know what i would use for code? Thanks