Stop European Date Format Changing

I have a userform which is used to update a spreadsheet.

There are 7 textboxes which add dates to the spreadsheet. These dates are then used in formulas.

Every time a record is saved the date format changes.

Ths means that if the date is entered dd/mm/yyyy it writes mm/dd/yyyy and this is fine. When this record is next retrieved it will show mm/dd/yyyy and any new date will be dd/mm/yyyy.

Wen saved to the spreadsheet both formats change meaning the dates in the spreadsheet appear in diffrent formats messing up all formulas.

How can I stop the date format changing each time the record is saved?