Find out how long the sql server service has been running, from t-sql -

I would like to know how long the SQL Server is running if it is possible to work from within the SQL server.

wants to experiment with one of the DMVs for the unused index, but the counter resets the SQL Server load every time, so I want to know how useful they are.

  select login_time from sys.dm_exec_sessions where session_id = 1  

will give you a datetime for the time the server is started.