First, fire your developer and store the time in a datetime field.

In the meantime, use this code:

declare @Start char(6), @End char(6)

set @Start = '133737'

set @End = '154040'

declare @StartTime datetime, @EndTime datetime

set @StartTime = stuff(stuff(@Start,3,0,':'),6,0,':')

set @EndTime = stuff(stuff(@End,3,0,':'),6,0,':')

select @StartTime, @EndTime, convert(char(8), @EndTime - @StartTime, 108)

After you've changed the fields to datetime, all you would have to do is:

select convert(char(8), EndDate - StartDate, 108)