Recently I had to come up with a way of determining how long a SQL Instance had been running. It turns out that there are a few ways to do it and here is my preferred way:

SELECT crdate

FROM sysdatabases
WHERE [name] = 'tempdb'

This works because the tempdb database is recreated every time the SQL Service is started. As an alternative you could get the date from the first entry in the current SQL Log:

SQL Log