Multiple True instructions for one If

I am struggling to figure this out. The scenario looks like this: Two columns with start and end date is present. I created 3rd to find the number of days. Pretty simple. But, when I did this, surprisingly, I got negative results for few rows meaning that some entries had end date before the starting date and this is serious mistake. So, what I was thinking of doing was to change the entries with negative results to "0" and mark the cell as red for reference to wrong entry. To do this, I have to check whether the cell value is less than 0. If TRUE, then assign value zero and fill the cell with red color but I am stuck here.

Any help will be great.

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