Same thing you are doing as adding 1h55m + 1h55m = 310
for i=1 to numberofoverflow=1
add 40 t0 310

result will be : 3h 50m

Example2 : 1h50m + 1h50m +1h50m = 450
for i=1 to numberofoverflow=2
add 40 to 450

result = 5h 30m


Now adding 40 in this case or substracting 60 where you have take minute parametere only .
No difference....