select
convert(varchar,datediff(hh,convert(datetime,[record time]) ,convert(datetime,[unload time]) ))
+ ':' +
convert(varchar,datediff(mi,convert(datetime,[record time]) ,convert(datetime,[unload time]) )%60)
+ ':' +
convert(varchar,datediff(ss,convert(datetime,[record time];) ,convert(datetime,[unload time]) )%60)
as Duration
From YourTable
Happy Coding!
:)