Dates won't format in Excel 2007

I have been an Excel user for many years and feel very comfortable with it. I recently did a clean install of my OS (Win XP) and reinstalled Office 2007 - all good.
When using one of my Excel spreadsheets I notice that the date column no longer converts my date input to a Date format. I used to put in 23/11 and that would be converted to 23/11/13 on leaving the cell. I have tried reformatting to the Date format I want and even tried again in a new workbook. What have I missed?