Conditional Formatting - Set colour based on date past

I have a spreadsheet that contains a long series of dates in Column B. I need to know how to set Conditional Formatting so that if any date is 180 days before today, it turns green. If any date is 270 days before today it turns yellow, and if any date is more than 270 days before today, it turns red. I know this may be a simple question, but I'm stuck on this! Thanks in advance.