Highlight a cell when there's a date clash

Apologies for the somewhat cryptic title, but I found it difficult to shorten it and still keep it meaningful...

A friend manages a cottage he and some other people own together.

Whenever someone wants to use the cottage for a few days he enters the arrival and departure date in two cells in adjacent columns. He normally sorts the columns on the arrival date to prevent scheduling issues. He recently made two mistakes on the same day when he didn't sort the columns.

Is there a way how either/both cells can be highlighted if either fall within any of the periods in the rows above?

Thanks a lot!

Sure. How depends on the layout of the data. (Hint: Post a workbook.)

Sure, attached an example that will hopefully explain the situation.

It might also be useful if it highlights the previous data the new date(s) clash with

Yes sorting it will help, but he wants it to be "octogenarian-proof"...

In G2 and copy down, =SUMPRODUCT( (D2 < E$1:E1) * (E2 >= D$1:D1)) When the result is non-zero, there's a conflict somewhere above, which you can detrmine by sorting.

Or you can use that formula for conditional formatting.