Visual Basic

Dim M9 = AmCommon.MakeMonthStartDate(-8)
Dim M10 = AmCommon.MakeMonthStartDate(-9)
Dim M11 = AmCommon.MakeMonthStartDate(-10)
Dim M12 = AmCommon.MakeMonthStartDate(-11)

Dim sM2 = AmCommon.MakeMonthStopDate(-1)
Dim sM3 = AmCommon.MakeMonthStopDate(-2)
Dim sM4 = AmCommon.MakeMonthStopDate(-3)
Dim sM5 = AmCommon.MakeMonthStopDate(-4)

Public Shared Function MakeMonthStartDate(byVal monthsBack as Integer) As DateTime

    Dim month = DateTime.Now().AddMonths(monthsBack)
    Dim fd = FirstDayOfMonth(month)
    Return New DateTime(month.Year, month.Month, fd.Day, 0, 0, 0)

End Function
Public Shared Function MakeMonthStopDate(byVal monthsBack as Integer) As DateTime

    Dim month = DateTime.Now().AddMonths(monthsBack)
    Dim ld = LastDayOfMonth(month)
    Return New DateTime(month.Year, month.Month, ld.Day, 23, 59, 59)

End Function

Public Shared Function FirstDayOfMonth(ByVal sourceDate As DateTime) As DateTime
    Return New DateTime(sourceDate.Year, sourceDate.Month, 1)
End Function

Public Shared Function LastDayOfMonth(ByVal sourceDate As DateTime) As DateTime

    Dim daysInMonth = DateTime.DaysInMonth(sourceDate.Year, sourceDate.Month)
    Return New DateTime(sourceDate.Year, sourceDate.Month, daysInMonth)

End Function

Dim Today As Date = Date.Today
Dim M1 As Date = Today.AddDays(1 - Today.Day)
Dim M2 As Date = M1.AddMonths(-1)
Dim M3 As Date = M1.AddMonths(-2) ' Or: M2.AddMonths(-1)
...

• >= 2020/02/01 And < 2020/03/01
• >= 2020/01/01 And < 2020/02/01
• >= 2019/12/01 And < 2020/01/01
• etc.