If a spreadsheet has a blank cell, the date value that I would expect it to be is 12/30/1899. At least, that is what Sheets would evaluate it as. When I use some date functions in spreadsheet.com on blank cells, the DAY function’s result is an error instead of 30. For example,
the formula =YEAR(B14) returns 1899
the formula =MONTH(B14) returns 12
the formula =DAY(B14) returns #VALUE!