Highlight Overdue Due Date

I have attached a sheet that I need a little bit of help with.

In column A, I enter the start date and it calculates in column C, E, G specific due dates for certain tasks. When the date has passed, I want it to highlight the cell RED to show it is past due.

In columns D, F, H, I have a drop down to mark once it is completed and when this is selected I want the cells in columns C, E, G to highlight GREEN to show it have been done.

The conditional formatting for the second part to mark it Green I have figured out, I just can't seem to get the highlighting it RED when the due date has passed to work.