Hi, the plugin doesn’t do any date parsing. It passes the post dates as string to WordPress. So, the date formats are those allowed by WordPress.
In my opinion the best format is always “yyyy-mm-dd hh:mm:ss” because it’s unambiguous. You can leave off the time “hh:mm:ss” if you’re just interested in dates.
I think WordPress uses strtotime() and will thus accept a lot of variations:
Excel does use an internal format for dates, but when you save an Excel file as .CSV I think it converts the dates to strings automatically.