Highlight cells on 20 days from birthday

I have an excel spreadsheet with about 50 names. One of the columns on the spreadsheet is labeled, "Birthday." I am trying to figure out how I can program this spreadsheet to cause the cell to change it's background color so that when I glance at the spreadsheet I can tell whose birthday is 20 days out from the day that I am looking at the spreadsheet. In other words, today is May 11th. I would like to open up the spreadsheet today and have any birthday 20 days or less out from today have the cell highlighted so that I can easily pick out whose birthday is coming up. Then after the birthday date is over, have the cell go back to it's normal color.