As Mike says, dates are only cosmetic.
You could include code in the 'Before Close' event which reformats all dates in your desired format, but that might result in errors as users in Australia etc. assume that dates are in the logical day-month-year order, rather than the US standard month-day-year.
You could add two buttons with some code.
One button formats all dates per US standard and the other button formats all dates per day-month-year format.
This sample code shows that it would not be too complicated to do, then wherever the spreadsheet is opened the user can see dates in their preferred format:
Dim rngCell As Range
For Each rngCell In Worksheets("Sheet1").UsedRange
If IsDate(rngCell.Value) Then
rngCell.NumberFormat = "mm-dd-yy"
The sheet name will need to be changed to match yours.
Also the code would need to be altered if more than one sheet in the workbook has dates in it.