Excel 2007 : Excel 2007 - X-Axis problem

I'm not a novice but am new to 2007 and have seen something odd that I can't figure out.

I’ve copied a chart that shows last 12 month performance from one Excel workbook to another
The chart is perfect in both workbooks until I close the original source workbook
When I close the original workbook the 2010 dates on the X-axis on the copied chart revert to Excel date format (40161, 40168 etc)
The dates for 2009 on the same chart are absolutely fine in dd/mm/yy format
I’ve tried formatting the X-axis but it doesn’t change a thing
If I re-open the original workbook the chart goes back to normal

Have you ever seen this before? Any suggestions as to what it might be?