Ok, after opening my eyes ;-)
It's strange you don't see the exception throw by isDateTime : this mehod want a cell (object) not a cell coordinate, so rewrite your test using $cell (current cell object if i'm correct).
Next, you have "in your hand", a date formatted in your locale (i guess) but if your MySQL field is date's type, it's better to have a string formatted accordingly for your insert, so format the "number" using FORMAT_DATE_YYYYMMDD2.