Dax conditional If statement for counting rows in date table

here's a general idea of how to implement that

ct2 =
VAR a =
    SUMMARIZE(
        MMYYDD,
        MMYYDD[MMYYDD],
        "ct",
            VAR d = [MMYYDD]
            RETURN
                CALCULATE(
                    COUNTROWS( Summary ),
                    COALESCE( Summary[RejoinDate], Summary[MinDate] ) <= d,
                    COALESCE( Summary[DateEnd], Summary[MaxDate] ) > d
                )
    )
RETURN
    SUMX( a, [ct] )

You can refine the logic by testing for both conditions together.