You should be able to just subtract SLA_Time from END_Time, but I see that your SLA_Time uses a different date than the END_Time timestamps. I'm guessing you want to assume that SLA and END are on the same date.
In your example you are using the time() function to extract the time portion for the calculation. That won't work. Time() is a formatting function. It only affects display values, not the internal value used in the calculation.
You can extract the time portion for arithmetic using frac(). So try:
frac(SLA_TIME) - frac(END_TIME)
and format as an interval in the Number pane.