According to the ERC Work Programme 2020, the date of the first PhD considered for the calculation of the eligibility period shall be the date of the actual award according to the national rules in the country where the degree was awarded. The Information for Applicants document to the Starting and Consolidator Grant 2020 calls complements that this is "generally, the date of successful defense/viva". In case of applicants having been awarded several PhDs, the reference date is the award date of the first PhD. In case of degrees in medicine considered equivalent to a PhD, the certified date of the medical degree completion plus two years is the time reference for calculation of the eligibility time-window.
For more information, see section 'Eligibility criteria' of the ERC Work Programme 2020 and its Annex 2.