jamex wrote:
How to set default format as DD/MM/YY in entire workbook.
Whenever I open new workbook, Date must be accepted in DD/MM/YY
format.

Excel takes its date format from Windows. I typed "excel default date
format" into google and found http://tinyurl.com/lmj9y as the third entry.
This tells you how to change the default date format.

HTH