EDIT: I needed to add a little more to the formulas for if the due date was blank and to avoid blue warning triangles.
If the duration in the "days overdue" cell is for display purposes only and will not be used as a duration value in a downstream formula,
=IF(OR(B1="",B2=""),"",IF(B2>B1,B2−B1,""))
If the value will be used in downstream formulas that expect it to be a duration, a null string from the above formula will create an error. One option is
=IF(OR(B1="",B2=""),DURATION(0),IF(B2>B1,B2−B1,DURATION(0)))
or
=IF(OR(B1="",B2=""),DURATION(0),MAX(B2−B1,DURATION(0)))
then set up a "custom highlight" of "when duration equals 0ms, set text opacity to 0" which will make it invisible. The text opacity can be set by clicking on the color wheel to the right of B I U S to bring up the color wheel then dragging the opacity slider to zero.
