=TEXT(A1,"hh:mm") & "-" & TEXT(B1,"hh:mm")
Anytime you you use the CONCATENATE function or Concatenate operator (&) the result will always be text.
The issue you have is that Excel stores dates and times as numbers, so you have to convert the time to Text prior to concatenating the time values or you will get the text version of the number that represents that time.
Does that make sense?
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