Get Counts in one table with related records in another table filtered by dates

[Anonymous]

I have two tables A and B as shown below. The AccountID in A has a relationship with the AccountID in B. 

A

AccountID CmpName AccFlag SysStartTime
A1 Test1 1 1/1/2020
A2 Test2 0 1/2/2020
A3 Test3 1 1/2/2020

B

ContactId AccountID ConFlag SysStartTime
C1 A1 1 1/1/2020
C2 A1 1 1/1/2020
C3 A1 0 1/1/2020
C4 A2 1 1/2/2020

I want to get the count of records in A that have 3 related records in B in a measure. The record A1 in A has 3 records with A1 in B. So the count returned should be 1. I want to filter the results based on the sysStartTime of A and B as well. For example, I want to get the count of records in A that have 3 related records in B as of 1/1/2010. I tried many things but I can't get the DAX for this. Please advice. 

[Stachu]

I'd start with adding Calendar table and AccountID table, to use following schema:

M code for AccountID based on A & B:

let
    Source = #table({"AccountID"},{{List.Distinct(List.Combine({A[AccountID],B[AccountID]}))}}),
    #"Expanded AccountID" = Table.ExpandListColumn(Source, "AccountID")
in
    #"Expanded AccountID"

then this measure will work:

Measure =
VAR __B_WithCount =
    ADDCOLUMNS ( 'AccountID', "Count", COUNTROWS ( RELATEDTABLE ( 'B' ) ) )
VAR __B_Filtered =
    FILTER ( __B_WithCount, [Count] = 3 )
RETURN
    COUNTX ( __B_Filtered, [AccountID] )