This is assuming that column a is start time, column b is end time, and column c is blank. first, format columns a and b as special, and type in hhmm as your type. Format column C as text.<--(important) If you type in 8:00 am into column a it should switch it to 0800. if you type 4:00 pm into column b it should show up as 1600. Now, hit Alt-F11, double click sheet 1 and paste the code below. I would try this with a test sheet first as there is no undo for vba code. This worked on my end for what you wanted to do.
For i = 1 To 200
starttime = Cells(i, 1).Text
endtime = Cells(i, 2).Text
Cells(i, 3) = starttime & endtime