Goussing is right, this ansewr is based on his (i upvoted his answer), i did this on D7, steps:
1.- convert your date to unix (https://www.extendoffice.com/documents/excel/2473-excel-timestamp-to-date.html), (i did this in libreoffice, same thing) this way no matter wich language is your drupal site and date formatting.
2.- go to your node display settings,...
I've struggled with this question today, and because i didn't found any clear answer here's the method with drupal 8.7:
go to your node type structure, and select the display of your field (for example : DD-MM-YYYY)
go back to the feeds mapping, remove your date entry.
add a new entry for the date, or rebuild the entire mapping if you can't change the order ...