join_date in ExpressionEngine is generally formatted as a UNIX timestamp and looks something like
field type length value
join_date INT 10 1118111578
It looks like your date is being converted to a human readable format on export, but you need to keep it as a UNIX Timestamp, for it to get stored properly.
I can't say, how much trouble maybe caused by the lack of a join_date or a join_date of 0. Anything that uses the
join_date to calculate or display would be boinked, but in our installs, the join_date isn't used that much. Just to look-up when we added someone, if the were giving control panel access. So someone else would need to chime in on that component.
I believe Export It was designed to make your export Excel compatible, which is different from exporting data and re-importing it back into EE.