Vista reports running on EXCEL 2010 do not save the date format correctly.
Microsoft EXCEL 2010
Vista 5.1.5 and below
Excel 2010 is not supported on Vista 5.1.5 and below
To get the date and time format to work correctly with Vista uninstall Excel 2010 and install Excel 2007.
The other option is to upgrade to Vista 5.1.7 which supports Excel 2010
A possible workaround is to insert your trends into the Work Area of the report. That is the area to the right of the main page.
Then you can copy this information to the main page and format the cells on the main page.