CSV Automatically Corrects My Date Format

I'm using a Excel spreadsheet to add in info to import into a website.

On the website, the date format MUST be yy-mm-dd to import correctly. The file must be saved as a CSV.

I've tried changing the cell format to text, changing it to custom, etc which works fine, until I save the file as a CSV. I then open up the CSV, and it's changed the format to dd-mm-yyyy. I have absolutely no idea how to stop this, because the CSV seems to disregard the cell format and correct it anyway. 

Is there any way I can KEEP the format as yy-mm-dd without the CSV changing it?