Sub A()
Dim qDate As Date
Dim LValue As String, RValue As String
    Select Case DatePart("q", Now)
        Case  2 
            qDate = DateSerial( 2009 ,  3 ,  1 )
        Case  3 
            qDate = DateSerial( 2009 ,  6 ,  1 )
        Case  4 
            qDate = DateSerial( 2009 ,  9 ,  1 )
        Case  1 
            qDate = DateSerial( 2010 ,  1 ,  1 )
    End Select

Debug.Print "LValue: " & qDate
RValue = (DateAdd("m", - 1 , qDate))
Debug.Print "RValue: " & RValue
End Sub

Sub q(mydate As Date)
Dim firstday As Date, lastday As Date, quartback As Date

    quartback = DateAdd("q", - 1 , mydate)
    firstday = DateSerial(Year(quartback), (Month(quartback) \  3 ) *  3  -  2 ,  1 )
    lastday = DateAdd("q",  1 , firstday) -  1 

    Debug.Print firstday, lastday
End Sub

Sub q(mydate As Date)
Dim firstday As Date, lastday As Date, quartback As Date

    quartback = DateAdd("q", - 1 , mydate)
    firstday = DateSerial(Year(quartback), DatePart("q", quartback) *  3  -  2 ,  1 )
    lastday = DateAdd("q",  1 , firstday) -  1 

    Debug.Print mydate, firstday, lastday
End Sub

Sub q(mydate As Date)
Dim firstday As Date, lastday As Date, quartback As Date

    quartback = DateAdd("q", - 1 , mydate)
' получили дату ровно на 1 квартал назад от заданной
    firstday = DateSerial(Year(quartback), DatePart("q", quartback) *  3  -  2 ,  1 )
' от полученной даты берем год, и номер её квартала
' номер квартала преобразуем в номер месяца так: 1 ->1 ; 2 ->4 ; 3 ->7 ; 4 ->10 (с помощью арифметики)
    lastday = DateAdd("q",  1 , firstday) -  1 
' добавляем ровно 1 квартал без 1 дня - получаем последний день квартала

    Debug.Print mydate, firstday, lastday
End Sub