Format your answer cell as time, showing hours, minutes and seconds
(the custom format looks like h:mm:ss;@). Then enter this formula:
=(DATE(MID(B1,1,4),MID(B1,5,2),MID(B1,7,2))+TIME(M ID(B1,9,2),MID(B1,11,2),MID(B1,13,2)))-(DATE(MID(A1,1,4),MID(A1,5,2),MID(A1,7,2))+TIME(MI D(A1,9,2),MID(A1,11,2),MID(A1,13,2)))

This formula parses the date/time stamp string into date and time
formats recognized by Excel, then subtracts the start from the finish.