You can change the date and time format. It's just under the Document settings rather than the specific tag settings. Though this then becomes an account wide change.
Admin can Define the Document Settings and adjust the "Date Signed Format".
Please view the guide on Administer Your Account Guide.
The steps are select
Go to Admin >> Signing and Sending >> Signing Settings >> Document Formatting >> Select Format
It is under: Preferences >> Account Administration >> Features >> Sending and Signing >> Format for Date Signed (dropdown)
**Please note that this is an account wide setting that must be enabled by your Account Admin. If you cannot view, then you may need to contact your Admin.
Adding to Paul's answer. If your documents are already out for signature before you set the "Date Signed" Tag you can log into DocuSign and CORRECT THE ENVELOPE, to add those tags while the envelope is still in process. Any roles and their associated tags that have been completed when you login to correct cannot be changed.
The Certificate of Completion does contain the time-stamp of the signature as well as the date. This is created whenever an envelope is completed.
Hope this helps.