Unfortunately, SQL Server does not provide an easy and simple query that answers this question. What we could do is use the fact that tempdb database is being created every time we start SQL Server and query the create time of this reincarnating temporary database. The table/columns we are interested is sysdatabase.crdate The following statement demonstrates it. Just copy it and run it in SSMS.

(I have written it for SQL 2005 but I guess it runs both on 2000 and 2008 versions)

declare @startupDate datetime
,		@days bigint
,		@hours bigint
,		@minutes bigint
,		@seconds bigint

select	@startupDate = CrDate
,		@seconds = DateDiff(second, CrDate, getdate())
,		@days = @seconds/60/60/24
,		@seconds = @seconds - (@days*60*60*24)
,		@hours = @seconds/60/60
,		@seconds = @seconds - (@hours*60*60)
,		@minutes = @seconds/60
,		@seconds = @seconds - (@minutes*60)

from	sysdatabases (nolock)
where	[name] = 'TempDb'

select	@startupDate startup_time
,		cast(@days as varchar) + ' days ' +
		case when @hours < 10 then '0' else '' end + cast(@hours as varchar) + ':' +
		case when @minutes < 10 then '0' else '' end + cast(@minutes as varchar) + ':' +
		case when @seconds < 10 then '0' else '' end + cast(@seconds as varchar)
		as online_duration