this is what i've used for quite a long time:

I wanted both server uptime and the SQLService uptime.

i made a simple stored proc, sp_help_uptime and put it in master, that returns results form a dmv:

--Results:

ServerRestartTime ServiceRestartTime ServiceUpTimeInDays ServerUpTimeInDays

2016-10-15 00:20:34.153 2016-11-11 10:15:24.023 7 34

the proc:

IF OBJECT_ID('[dbo].[sp_help_uptime]') IS NOT NULL

DROP PROCEDURE [dbo].[sp_help_uptime]

GO

CREATE PROCEDURE sp_help_uptime

AS

select

DATEADD(second, -1 * ( ms_ticks / 1000),getdate()) as ServerRestartTime,

i.sqlserver_start_time As ServiceRestartTime,

DATEDIFF(dd,i.sqlserver_start_time,GETDATE()) AS ServiceUpTimeInDays,

DATEDIFF(dd,DATEADD(second, -1 * ( ms_ticks / 1000),getdate()),GETDATE()) AS ServerUpTimeInDays

from sys.dm_os_sys_info i;

GO