User setting changing date format? Possible? MM/DD/YY vs DD/MM/YY

I have a dozen or so users that fill out the same form that includes a drop down. The dropdown is populated with a list of dates using .toLocaleDateString(). One user’s input is being entered into the database as DD/MM/YY. Everyone else who uses the same drop down, the date is submitted as MM/DD/YY. Is there something on the user’s side that is affecting this? If not, where would I start looking to edit this?

I’m using a Wix form to capture the data.

Do you save the selection as Date&Time type? String? Number?
Please post your code.