Fix Excel Date Formatting

I have a worksheet built in Excel 2010 where all dates will be typed in by hand. I need every number that represents a date to appear automatically as DD/MM/YYYY, regardless of how the user writes it during entry. A clean, reliable solution that works natively in Excel 2010 is essential—power-query or dynamic array techniques from newer versions won’t run here.

Please adapt the file (or provide a small VBA routine, custom format, input mask, or data-validation rule) so that:

• any date input is instantly shown as DD/MM/YYYY
• no accidental text or U.S.-style MM/DD confusion can slip through
• the fix remains stable when the workbook is shared between different PCs running the same version

Deliverables:

• updated workbook or code module ready to drop into my file
• a brief “how it works” note so I can maintain it later

Once I confirm the formatting behaves exactly as described, the job is done.