answering my own question, by converting ms_ticks to seconds, i think i see what i was after:

--ServerRestartTimeServiceRestartTimeServiceUpTimeInDays

2015-08-22 21:46:51.9802016-03-29 08:34:07.89087

-- 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

from sys.dm_os_sys_info i;