Having said this, the question remains how applicants in doubt can find out which is the correct date in the case they have two dates on their diploma. The solution is to contact the doctoral office of the university where they obtained their PhD. Normally every university has a regulation that clarifies among other points which date entitles the applicant to have the title of “PhD”. These regulations can often be found online.
In Switzerland, as in other countries as well, there are no national rules on the PhD but such points are typically ruled by the universities.
Note that it can happen that the ERC contacts the applicant after the eligibility check, informing him/her that s/he is not eligible, just because the ERC staff chose the wrong date on the diploma. In this case the applicant shall explain the situation and submit the PhD regulation in order to show that s/he is indeed eligible. The time frame to submit an answer can be quite short.