Mandatory Cells

I've done a formula to add "date required" when the box is checked and " " (blank) when date is not checked.

Issue is when the user puts the date in the field, it removes the formula, so if they unchecked the check box, or remove the date, the date remains blank.

Although I have formatted the cell to stay Red if blank, I'm interested to know if there is a way of making the "date required" return when the cell is blank again automatically?

This is simply say if someone checked the box by mistake or removed the date by mistake, they would be reminded that this was not complete..