When is the official date a degree is obtained?
Usually the graduation date is the date on which the last accomplishment needed for graduation was completed.

This could be a thesis defense but could also be something else. For example in some Master and Bachelor programs I know one may have a "final" presentation prior to handing in the thesis. So the graduation date is the date at which the thesis is handed in. Note that even though a thesis defense may not count towards your grade, it is usually still a required accomplishment and consequently it date counts.