Which method are you using to copy the date column,
if read range is used to read that column and write range to write into another excel I would suggest you to check the option “Preserve Format” during the read range
If invoke code is used then in pastespecial method use,
Pastespecial (pasteallusingsourceformat)
if it is done manually then while pasting the range do a right click and there will be paste types select pasteallusingsourceformat
Use Excel Scope for both reading and writing. Do not use Workbook activities.
For Reading → Excel Scope-> Read Range
For Writing → Excel Scope → Write Range
Have to use the Date.Parse(dateString, "dd/MM/yyyy", Nothing) also