The formula is simple. You add the two numbers and divide them by 2 and you have the mid value.
But the problem here is the numbers here are in time format. Moreover there could be AM/PM issues.
So this is what you do here:
STEP 1: Convert Time1 to milliseconds
STEP 2: Convert Time2 to milliseconds
STEP 3: Result_In_MilliSeconds = (Result_From_Step1 + Result_From_Step2) / 2
STEP 4: Convert Result_In_MilliSeconds into required time format.
I will leave syntax part for you to do. There is help on Google on how you do these conversions.