select 
  sqlserver_start_time  
from sys.dm_os_sys_info

select
	[OS Start Time]	= convert(varchar(23),b.OS_Start,121),
	[SQL Server Start Time]	= convert(varchar(23),a.SQL_Start,121),
	[SQL Agent Start Time]	= convert(varchar(23),a.Agent_Start,121),
	[OS Uptime] =
		convert(varchar(15),
		right(10000000+datediff(dd,0,getdate()-b.OS_Start),4)+' '+
		convert(varchar(20),getdate()-b.OS_Start,108)),
	[SQL Uptime] =
		convert(varchar(15),
		right(10000000+datediff(dd,0,getdate()-a.SQL_Start),4)+' '+
		convert(varchar(20),getdate()-a.SQL_Start,108)) ,
	[Agent Uptime] =
		convert(varchar(15),
		right(10000000+datediff(dd,0,getdate()-a.Agent_Start),4)+' '+
		convert(varchar(20),getdate()-a.Agent_Start,108))
from
	(
	Select
		SQL_Start = min(aa.login_time),
		Agent_Start =
		nullif(min(case when aa.program_name like 'SQLAgent %' then aa.login_time else '99990101' end),
			convert(datetime,'99990101'))
	from
		master.dbo.sysprocesses aa
	where
		aa.login_time > '20000101'
	) a
	cross join
	(
	select
		OS_Start = dateadd(ss,bb.[ms_ticks]/-1000,getdate())
	from
		sys.[dm_os_sys_info] bb
	) b

OS Start Time           SQL Server Start Time   SQL Agent Start Time    OS Uptime       SQL Uptime      Agent Uptime
----------------------- ----------------------- ----------------------- --------------- --------------- ---------------
2012-08-04 22:19:26.317 2012-08-04 23:38:07.163 2012-10-31 11:49:49.063 0100 18:28:38   0100 17:09:57   0013 04:58:15

We walk in the dark places no others will enter
We stand on the bridge and no one may pass