Q:

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

I'd like if its possible to work out from inside sql server how long sql server has been running.
Would like to use this in conjunction with one of the DMV's for unused indexes, but the counters are re-set every time sql server loads, so I'd like to know how useful they're going to be.

A:

SELECT
    login_time
FROM
    sys.dm_exec_sessions
WHERE
    session_id = 1

will give you a datetime for when the server was started.

A:

To get it programmatically, you can run this script. It checks the creation time of your tempdb, since tempdb gets reinitialized every time Sql Server is started.
SELECT create_date 
FROM sys.databases 
WHERE name = 'tempdb'

To make it more intuitive, you can run the script below, which will tell you how many days and hours Sql Server has been running. Minutes and seconds information will be truncated. If you need that, modify the script to get it yourself.
SELECT 'Sql Server Service has been running for about '
+ CAST((DATEDIFF(hh, create_date, GETDATE()))/24 AS varchar(3)) + ' days and '
+ CAST((DATEDIFF(hh, create_date, GETDATE())) % 24 AS varchar(2)) + ' hours'
FROM sys.databases 
WHERE name = 'tempdb'

Source: How long SQL Server has been running

