Calculating Repayment Rate Using Excel

Question

Goal: Considering the date of disbursement cohort, I need the month with the lowest repayment rate in 2018 (loans repaid on time)? Using pivot table

id	principal_disbursed	principal_repaid	due_date	repayment_date	credit_officer
1	$334	$334	12/10/2017	17/10/2017	John
2	$184	$184	10/11/2018	20/10/2018	Maria
3	$417	$417	18/08/2017	24/08/2017	Maria
4	$500	$500	04/08/2017	14/07/2017	Maria
5	$250	$250	04/08/2017	06/08/2017	John
6	$250	$250	02/08/2017	24/07/2017	Frank
7	$500	$500	04/08/2017	12/08/2017	John
8	$417	$417	10/08/2017	08/08/2017	Maria
9	$334	$334	05/08/2017	09/08/2017	Frank
10	$500	$500	01/08/2017	05/08/2017	Maria
11	$400	$400	08/08/2017	10/08/2017	Frank
12	$500	$500	10/08/2017	29/07/2017	Frank
13	$500	$500	10/08/2017	14/08/2017	John
14	$500	$500	12/08/2017	30/07/2017	Frank
15	$484	$484	04/08/2017	07/08/2017	Maria
16	$167	$167	13/08/2017	14/08/2017	Frank
17	$334	$334	09/08/2017	22/08/2017	John
18	$450	$450	06/09/2017	05/09/2017	Maria
19	$500	$500	09/09/2017	17/09/2017	Maddy
20	$250	$250	23/08/2017	16/08/2017	John
21	$250	$250	07/09/2017	16/08/2017	Frank
22	$167	$167	10/09/2017	12/09/2017	Maria
23	$500	$500	15/08/2017	17/08/2017	Maria
24	$250	$250	12/08/2017	06/09/2017	Frank
25	$500	$500	13/08/2017	19/08/2017	Maria
26	$250	$250	13/09/2017	21/08/2017	Maria
27	$250	$250	14/09/2017	03/09/2017	Frank
28	$500	$500	16/09/2017	06/09/2017	John