Explanation
Consider for a moment how overlapping dates work.
The SUMPRODUCT function is perfect for this kind of test because it handles array comparisons elegantly.
Conditional formatting dates overlap
To highlight cells where dates overlap you can use conditional formatting with a formula based on the SUMPRODUCT function. In the example shown the formula in cell E6 is: =SUMPRODUCT(($C6=$C$5:$C$9))>1 This is the same formula used to highlight entire rows in the table using a formula-based conditional formatting rule.
