Date format and difference in time

All you would need to do is =B3-C3, if B3 is your converted end date, and C3 is your converted start date.
If you wanted one formula to show you the difference, referencing the original unformatted dates, you could use the following (B2 is the end date, C2 is the start date):

Formula:

=(DATE(LEFT(B2,4),MID(B2,5,2),MID(B2,7,2))+TIME(MID(B2,10,2),MID(B2,13,2),MID(B2,16,2)))-DATE(LEFT(C2,4),MID(C2,5,2),MID(C2,7,2))+TIME(MID(C2,10,2),MID(C2,13,2),MID(C2,16,2))