The times they are a changin'. Or so wrote a Nobel Laureate a while back. We've also just moved from the 20th to the 21st Century.
It also means that importing dates is now a tough task with only two digits (e.g. dd-mm-yy) because did you mean 1916 or 2016? And even Excel gets confuzzled sometimes and guesses with leading zeros. It doesn't go well.
Meanwhile, back in TidyHQ, you're trying to import your date formatted list.
The only safe way to import is to use all four digits in the date format.
Therefore, instead of dd-mm-yy you will need format the date field to be dd-mm-yyyy to avoid any confusion.
See also: http://support.tidyhq.com/contacts/date-format-incorrect-on-csv-contacts-import-upload