Fix#2373. Excel can handle DateTime/Date/Time as a string if the datatype of the cell is set to "d". The string is, apparently, supposed to follow the ISO8601 spec. Openpyxl can be configured to generate a file with such values, so I've added support and set up unit tests. Excel, naturally, converts such a string input into its numeric representation of the date/time stamp. So will PhpSpreadsheet, so a call to setValueExplicit specifying Date format will actually see the cell wind up with Numeric format - there is no way (and no reason) for the Date type to 'stick'.
8ab834520d