Here are the following fields you will need to populate:
Next you need to format columns as depicted in the next three images
Here is what the data will look like when populated with the formulas that are below the image.
After entering each of the formulas in row 2, copy and paste to row 3 to the end of your data.
(paragraph added 2019-05-27 21:34) Note that column F "15 Minute Group ID" is the field that creates the 15-minute groupings. The last two digits of this field are "00","15","30", or "45". They represent the following minute groupings respectively, 0-14, 15-29, 30-44, or 45-59 minutes.
A2: =TEXT(YEAR(G2),"0000")
B2: =TEXT(MONTH(G2),"00")
C2: =TEXT(DAY(G2),"00")
D2: =TEXT(HOUR(G2),"00")
E2: =TEXT(MINUTE(G2),"00")
F2: =VALUE(A2&B2&C2&D2&TEXT(IF(VALUE(E2)<15,0,IF(VALUE(E2)<30,15,IF(VALUE(E2)<45,30,45))),"00"))
G2: =DATE(MID(H2,1,4),MID(H2,6,2),MID(H2,9,2))+TIME(MID(H2,12,2),MID(H2,15,2),MID(H2,18,2))
Now you need to create a Pivot Table.
Within the Pivot Table, add the following fields and then "Group" the "15 Minute Group ID" as shown in the next two images.
The Pivot Table will appear as follows after grouping.
Lastly, you can add fields to the table as you want to better illustrate you objectives.
If this solves your problem, please mark this as an answer.