Hi Emma,
I had the same problem, and it is maddeningly frustrating.
The solution I found was to create a column and force something to go in to the "Publish Up" field. This will then put something in that field instead of the bad text.
You must be extremely careful that whatever spreadsheet program you use (and libreoffice does handle it better than excel - i chased that problem for at least an hour) does not make the field into a date field with a different format. Go into cell formatting and override the cells to be text fields, and make sure the date is entered exactly YYYY-MM-DD HH:MM:SS or it will not work.
Disappointed in this bug, because as far as I can tell I've followed the instructions perfectly, but still a useful product at a reasonable price. Also, I beleive the creators follow
this forum
for tech support and so anyone who has issues should post there.
Mike