I have a survey data set where respondents DOB was entered as "DD/MM/YYYY". Sheets has formatted some of the cells as "MM/DD/YYYY", and where that wouldn't make sense because the day is greater than 12 it has formatted it as text. 

I want to change the "MM/DD/YYYY" formatting to "DD/MM/YYYY" formatting, so sheets treats them accurately in formulas, but I don't want it to change the actual values inside the cells as the way they are displayed are accurate. 

That is I have some date in a cell "05/10/2001". Sheets is treating that as the 10th of May 2001. The actual value is the 5th of October 2001. If I was to reformat it normally it would change the date in the cell to "10/05/2001". I want sheets to treat it like DD/MM/YYYY without actually changing the date in the cell.