I don’t think you want $start_date[$i]. You probably are looking for $start_date[0]. That’s why you’re only getting the first result and none of the following results. 1970-01-01T00:00:00 is the epoch time and shows up because strtotime() cannot convert the date you’re giving it, so that’s what it returns by default.
I would check out what $start_date looks like.