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