Need to calculate when a date is = or less than 30 days prior to today's date

I manage truck drivers and I need a way to be notified when their license and/or medical cards are within 30 days of expiration. For example, if a drivers license expires May 7, 2016 - I need the cell to turn red today (April 7, 2016). I have the formula =E3-30 to let me know what day is 30 days prior to expiration. But I need that cell to turn red when it's 30 days from any given "today". Hope I explained this okay...Thank you for any help.