Function ProjectSales(forMonth As Date, salesLastMonth As Integer, salesLastMonthYear As Integer, salesCurrentMonthYear As Integer) As Double
    Dim DC As Integer ' days in the current month
    Dim DL As Integer ' days in last month
    Dim DLY As Integer ' days in last month one year ago
    Dim DCY As Integer ' days in the current month one year ago

    DC = DaysInMonth(forMonth)
    DL = DaysInMonth(DateAdd("m", -1, forMonth))
    DLY = DaysInMonth(DateAdd("m", -13, forMonth))
    DCY = DaysInMonth(DateAdd("m", -12, forMonth))

    Dim SL As Integer ' sales in the last month
    Dim SLY As Integer ' sales in the last month one year ago
    Dim SCY As Integer ' sales in the current month one year ago
    SL = salesLastMonth
    SLY = salesLastMonthYear
    SCY = salesCurrentMonthYear

    Dim m As Double ' slope of the sales rate from the months last year
    m = ((SCY / DCY) - (SLY / DLY)) / ((DCY + DLY) / 2)

    ' our x = 0 is halfway through the last month.  Find the dates of the start and end of the current month based on our x
    Dim monthStart As Double
    Dim monthEnd As Double
    monthStart = DL / 2
    monthEnd = monthStart + DC

    ProjectSales = m * (monthEnd ^ 2 / 2) + (SL / DL) * monthEnd - (m * (monthStart ^ 2 / 2) + (SL / DL) * monthStart)
End Function