Date math in View table
You can change the value using a field template.


Add the date field to your view
Look in Advanced -> OTHER -> Theme: Information for a suitable file name for a field template that will only affect the field in this specific view.  They are listed in order from least specific to most specific so the final one in the list for that field is probably the best one to use.  The link on the filename will show you the default contents for such a file.
Create a file in your theme where you keep template files with that filename
Start with the default contents in that file and refresh your cache
Install devel module and use dpm($output) in your new template file to see the structure of the output
Use PHP to edit the appropriate portion of the output, you may need strtotime() and date_add() to achieve this.


Clear your cache often.  If you haven't already got drush installed, do it, it is a very useful tool.