Every time you start SQL Server "tempdb" gets re-created, so by seeing the tempdb creation datetime you can determine when SQL Server last started. Below query will return you the same.
SELECT name AS [Database Name], crdate AS [CreatedOn] FROM sys.sysdatabases WHERE name = 'tempdb'
Another way to see check the same is the SQL Server system view "dm_os_sys_info", which returns the information about the computer on which SQL Server is installed including the resources available to be utilised by SQL Server.
ms_ticks returns the number of milliseconds since the computer was started while sqlserver_start_time returns the date and time that SQL Server last started.
SELECT DATEADD(SECOND,((-1)*(DOSI.[ms_ticks]/1000)), Getdate()) AS OSStartedOn, DOSI.sqlserver_start_time AS SQLServerStartedOn
FROM sys.[dm_os_sys_info] DOSI