If you are looking for the date your domain controller was promoted in your domain, there is a easy way to find out that and it's showed at this post.
When a server is promoted to a Domain Controller, an "NTDS Settings" object for the server is created in the configuration container of Active Directory.
This object has a whenCreated attribute and it's possible to see it through the command below:
dsquery * "cn=Sites,cn=Configuration,dc=contoso,dc=com" -Filter "(cn=NTDS Settings)" -attr distinguishedName whenCreated
The output of this command will be something like that:
For any doubts or suggestions, please leave a comment below.