You need to run the following query and it will give you the date of your SQL Server Installation.
SELECT @@SERVERNAME SERVERNAME, CREATE_DATE ‘INSTALALTIONDATE’
FROM SYS.SERVER_PRINCIPALS
WHERE SID = 0X010100000000000512000000
SID 0X010100000000000512000000 is belongs to user "NT AUTHORITYSYSTEM". This user create at the time of installation only.