The IsValid property gets True, if the Expression property is syntactically correct. If the Expression property is not valid or is syntactically incorrect, the IsValid property returns False, The non-working time interval is visible ONLY, if the IsValid property is True. In other words, you can use the Expression to define whether a non-working time-interval is visible or hidden. For instance, you can add add an "A" character at the start of the Expression, and so the expression is not valid, and so no shown. NExt, remove the "A" character in front, and the non-working time-interval will be visible again.