We used to return -1 for invalid date input, but that will cause issue when customize the display.
E.g. If column A contains date, and user set column B’s formula to be Text(Month(A), "##")$, which means get month numbers from date in column A and format the display to show two digits, e.g. date in Jan will show 01. For invalid and empty date in column A, -01 will show.