Stop if the value is true + conditional formatting (mac-excel-2011)

[futrola]

hi guys

Please look at the attached picture, column C and D.

For column C I am using data validation drop down list (YES/NO) and conditional formatting (colors only) for each choice.

Column B automatically calculates number of days between the dates entered in column A and today's date.
Formula: =IF(A2-TRUE, TODAY()-A2)

What I want?
If I choose "yes" from the data validation list for C2, I want excel to stop calculating the number of days in B2, and immediately put a new date in D2 (date when C2's been chosen).
If I choose "no" from the data validation list for let's say C3, I want D3 cell to become red (i.e. same color as C3).

Of course, I want to apply those functions for rows between 2 and 20.
Is that possible?

Thank you in advance!