OOPS! Happened again - must be caused by the "less than symbol". Trying again.
better way would be to add 24 if the result is negative and leave it alone otherwise
So if c6 contains the start time and b6 the end time, then put the following formula in the cells and then use sum on these.
=IF((C6-B6) "less than" 0,C6-B6+24) In your example this would give 6-20+24=10 (just what you wanted!
Replace "less than" in the above expression by the less than sign on your keyboard.(SHIFT+.)
Then add them all up.
This would only fail if someone works a shift longer than 24 hours. Unlikely I hope!!