Counting The Number Of Days Overdue Based On Date And Time?

I have the following:

A1 = Date item is due. (DD/MM/YY format)

A2 = Time the item was delivered (HH:MM:SS format)

B1 = Actual Date item was delivered.

B2 = Actual Time item was delivered.

I'd like C1 to count the number of days the item was late. - taking into account that 16:00:00 is the deadline on each date.

For example:

Due Date: 17/06/14 @ 16:00:00

Arrival Date: 17/06/14 @ 16:01:00

Days Late = 1

Due Date: 17/06/14 @ 16:00:00

Arrival Date: 18/06/14 @ 15:00:00