If you're sure that all you need is HH:MM:SS, then this will work.

with cte as (

select datediff(second, '09:28:00', '13:38:02') diff)

select

right('00' + convert(varchar(2), (diff % 86400) / 3600), 2) + ':'+

right('00' + convert(varchar(2), (diff % 3600) / 60), 2) + ':' +

right('00' + convert(varchar(2), (diff % 60)), 2)

from cte;

If you want to calculate months, days, hours, minutes and seconds, then we'll need a different approach.