Excel pivot table problem - can't Group dates

i am currently using my own data to try out the the tutorial provided by excelzone in accountingweb.co.uk - 'Five minute's pivot table tip'.

i am trying to sort out the invoice listing into detailed ageing format. hence, for row - name, invoice number, column - date(dd/mmmm/yyyy), date - amount in GDP & USD.

so i try to change the date to month by following the tips given in excelzone. but message pop up and said 'cannot group that selection'.

i dont know where have i gone wrong. can anyone here please pinpoint out to me so that i can rectify it? thanks a lot
Ken Wen