Unless $fields_Array[$i] contains the name of an element in the $item[] array (which - and this can only be a guess based on what's been given - is the idea: presumably it's called "$fields_Array" because it's an array).
Assuming that, if strtotime() is being called with an empty parameter, and the only parameter is $item[$fields_Array[$i]], then logically $item[$fields_Array[$i]] is empty. And why that should be only the original poster is in a position to say.
(Incidentally, time() would be more straightforward than mktime().)