No, the only variable that the candidates can fill in is the [candidate_signature]. As for the rest of the e-signature variables (both default variables and custom variables), they are designed solely for the hiring team to fill in when completing the e-signature creation.
The [candidate_sign_date] variable is automatically populated by Dropbox Sign once each signatory signs the document, the candidate does not need to fill in this variable.
To find out when the candidate signed the e-signature if the [candidate_sign_date] variable is not included in the document, check the audit trail created after they signed.