Subindexing and Marginal Differencing

[dfdsiii]

I have this dataset:
[TABLE="width: 682"]
<tbody>[TR]
[TD]User[/TD]
[TD]Date[/TD]
[TD]Time[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/1/2019[/TD]
[TD]8:00 AM[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/1/2019[/TD]
[TD]10:00 AM[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/1/2019[/TD]
[TD]4:00 PM[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/1/2019[/TD]
[TD]5:00 PM[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/2/2019[/TD]
[TD]8:00 AM[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/2/2019[/TD]
[TD]5:00 PM[/TD]
[/TR]
[TR]
[TD]B[/TD]
[TD]9/1/2019[/TD]
[TD]9:00 AM[/TD]
[/TR]
[TR]
[TD]B[/TD]
[TD]9/1/2019[/TD]
[TD]3:00 PM[/TD]
[/TR]
[TR]
[TD]B[/TD]
[TD]9/1/2019[/TD]
[TD]5:00 PM[/TD]
[/TR]
</tbody>[/TABLE]
The first thing I want to do is add a subindex on User and Date.
[TABLE="width: 682"]
<tbody>[TR]
[TD]User[/TD]
[TD]Date[/TD]
[TD]Time[/TD]
[TD]User/Date Index[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/1/2019[/TD]
[TD]8:00 AM[/TD]
[TD]1[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/1/2019[/TD]
[TD]10:00 AM[/TD]
[TD]2[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/1/2019[/TD]
[TD]4:00 PM[/TD]
[TD]3[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/1/2019[/TD]
[TD]5:00 PM[/TD]
[TD]4[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/2/2019[/TD]
[TD]8:00 AM[/TD]
[TD]1[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/2/2019[/TD]
[TD]5:00 PM[/TD]
[TD]2[/TD]
[/TR]
[TR]
[TD]B[/TD]
[TD]9/1/2019[/TD]
[TD]9:00 AM[/TD]
[TD]1[/TD]
[/TR]
[TR]
[TD]B[/TD]
[TD]9/1/2019[/TD]
[TD]3:00 PM[/TD]
[TD]2[/TD]
[/TR]
[TR]
[TD]B[/TD]
[TD]9/1/2019[/TD]
[TD]5:00 PM[/TD]
[TD]3[/TD]
[/TR]
</tbody>[/TABLE]
The second thing I want is a column that takes the difference between times in its row (A) and the row after it (A+1). "n/a" means I don't really care about this value. I just want the interday marginal differences.
[TABLE="width: 682"]
<tbody>[TR]
[TD]User[/TD]
[TD]Date[/TD]
[TD]Time[/TD]
[TD]User/Date Index[/TD]
[TD]Marginal Difference[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/1/2019[/TD]
[TD]8:00 AM[/TD]
[TD]1[/TD]
[TD]02:00:00[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/1/2019[/TD]
[TD]10:00 AM[/TD]
[TD]2[/TD]
[TD]06:00:00[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/1/2019[/TD]
[TD]4:00 PM[/TD]
[TD]3[/TD]
[TD]01:00:00[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/1/2019[/TD]
[TD]5:00 PM[/TD]
[TD]4[/TD]
[TD]n/a[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/2/2019[/TD]
[TD]8:00 AM[/TD]
[TD]1[/TD]
[TD]09:00:00[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]9/2/2019[/TD]
[TD]5:00 PM[/TD]
[TD]2[/TD]
[TD]n/a[/TD]
[/TR]
[TR]
[TD]B[/TD]
[TD]9/1/2019[/TD]
[TD]9:00 AM[/TD]
[TD]1[/TD]
[TD]06:00:00[/TD]
[/TR]
[TR]
[TD]B[/TD]
[TD]9/1/2019[/TD]
[TD]3:00 PM[/TD]
[TD]2[/TD]
[TD]02:00:00[/TD]
[/TR]
[TR]
[TD]B[/TD]
[TD]9/1/2019[/TD]
[TD]5:00 PM[/TD]
[TD]3[/TD]
[TD]n/a[/TD]
[/TR]
</tbody>[/TABLE]

 

[Fluff]

Hi & welcome to MrExcel.
How about

Book1
	A	B	C	D	E
1	User	Date	Time
2	A	09/01/2019	8:00 AM	1	02:00:00
3	A	09/01/2019	10:00 AM	2	06:00:00
4	A	09/01/2019	4:00 PM	3	01:00:00
5	A	09/01/2019	5:00 PM	4
6	A	09/02/2019	8:00 AM	1	09:00:00
7	A	09/02/2019	5:00 PM	2
8	B	09/01/2019	9:00 AM	1	06:00:00
9	B	09/01/2019	3:00 PM	2	02:00:00
10	B	09/01/2019	5:00 PM	3
Data

Cell Formulas
Range	Formula
D2	=COUNTIFS(A$2:A2,A2,B$2:B2,B2)
E2	=IF(D3>D2,C3-C2,"")