Conditional format cell with date based on specific number

Given data:
O17 is a start date (8/29/2014)
P17 is estimated number of days to complete a project (56) ---this number will vary depending on project size.

What I am trying to accomplish:

Lets say the project started 8/29/14, it has been 42 days since project started, project was supposed to be done in 56 days. I want O17 to turn yellow to alert me that we are 14 days away from the project being due (P17-14)DAYS FROM O17).

At 7 days from when the project is due ((P17-7)DAYS FROM O17), or in this example 49 days after project has started, I would like O17 to turn orange.

On P17 days from start date and beyond....O17 should be red.

Is there an easy way to format this or am I making it more difficult? The reason I want it formatted based of of that given data (O17 AND P17) is because the actual end date may vary. Looking to make it more of a target date warning system.

Thanks for your help.

What you can do is in cell q17 put the formula =+P17+O17. Q17 would be the estimated last day of the project.

In cell R17 put +Q17-NOW(). That is the countdown of number of days left until the project is due.

Then go to Conditional formatting/Hilight cell rules while on cell R17 and do a conditional formatting from BETWEEN 7 and 14 as whatever color you want and then do another one for LESS THAN 7 for another color.