Field: Document Completion Date

[default6xqgrf55umcy]

Hello...I wonder that obviously nobody ever asked that yet. Is there a field, that can be added to the document, that represents the date on that the document was signed by all signatories (= completion date?). So far I only found the fields that show when a certain signatory signed the document. I would like to have a field that shows when ALL signatories have signed. 

Thank you in advance for your help!

[default6xqgrf55umcy]

...is there really no way to do this?