Calculate the count as per time difference using aggregate function in MongoDB

I need to calculate the no of records as per some time interval using MongoDB aggregate function. I am showing my collection below.

allocation:

state1                 state2                              createdDate

test1                  test2                     2019-08-16T09:41:22.202Z

test1                  test2                     2019-08-16T09:41:22.202Z

test3                  test4                     2019-08-16T09:41:22.202Z

test1                  test2                     2019-08-16T09:41:22.202Z                     

In my allocation collection I have state1,state2,createdDate column and here I need to calculate the no of record within some time interval as per user input (i.e-test1 & test2). Here Using aggregate function I need to the result like below.

difference                  count

0-6HRs                        2
6-12HRS                       0

................................
................................

The time difference will calculate from createdDate column value.

Copyright Notice：Content Author:「satya」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/57709087/calculate-the-count-as-per-time-difference-using-aggregate-function-in-mongodb