Clear direct formatting does not work, how do I treat the cell with a date in it as date and no as text

A cell that contains a date, 02/30/2020, is treated as text by my formula datedif from another cell whereas other cells with similar dates are treated as dates. Deleting the content of the cell does not help, neither does clearing direct formatting, nor does selecting the cell and clicking and Format>Number>Date. Other dates align to the right of the cell whereas this date aligns
to the left. Strangely, when I change the date from 02/30/2020 to 03/30/2020 then the content of the cell align to the right and is treated as date – and datedif computes correctly the difference. What is the reason of this issue? How might I solve it?