What is the difference between D/A and D/P?

At first, there're two party we need to notice:
1.The collecting bank 
2.The importer 

D/P means document against payment.
The collecting bank will pass relevant documents and receipts to the importer until the importer fulfil his/her payment.
D/A means document against acceptance.
The collecting bank will pass relevant documents and receipts to the importer as long as the importer promise to fulfill the payment before a certain date .

So, the difference between the two is the promising date, which implies that the importer can pay at any time as he/she want under D/A, also means the collecting bank should bare a bigger risk. Most importantly,the importer is legal to delay his/her payment.