IssuedOn meant for blockcert or for underlying award?

The issuedOn property in a blockcert seems to come from OpenBadges, where it is described as (from

issuedOn [DateTime] Timestamp of when the achievement was awarded.

Should issuedOn describe when the blockcert was issued, i.e., the date when the blockcert was logged on the blockchain, or should it describe the date on which the underlying credential was awarded?

So, say a Bachelors degree was awarded Mar 4, 2019 during convocation (i.e,. paper diploma given out), but the blockcert for that same degree wasn’t issued until Mar 8, 2019 – which date should be listed in issuedOn?

Thank you.