How to calculate movement

I have this spreadsheet (about 25000 rows) too many to do manually.
There are 3 columns - dd/mm/yyyy - hh:mm:ss - Direction
In the direction column there are 2 options BA and AB.. These are traffic counters..

So I would like some help with calculating the time it took for a car to drive over these counters.. When the time is more than 20 seconds, the car went into a yard.
However as you can see from the attached spreadsheet it isn't a simple formula. Well for me anyway.

So can someone help me with calculating the time, bearing in mind the different directions.

Thanks heaps