Excel 自定义函数更新不正确

问：

我正在尝试编写（我的第一个）自定义函数，该函数基本上将遍历报告以求和未来 1、3、6 个月（等）的机器成本

该函数返回值，但是，这些值不正确，更成问题的是，在包含自定义函数的单元格上加倍会更改使用自定义函数的周围单元格的值。

我正在处理的代码在这里：

Option Explicit

Dim months As Integer 'a month is considered as 30 days
Dim cost As Long
Dim FleetData As Range
Dim rowCounter As Long
Dim lastRow As Long
Dim firstRow As Long
Dim component As Range
Dim dateOfAction As Range
Dim totalApprox As Range
Dim dateHorizon As Date 'Date to which the user wants to total the maintenance cost for

Private Function totalCosts(xMonths As Range)
'Dim totalCosts As Long
Application.Volatile

dateHorizon = Date + (30 * months)

firstRow = [A10].Row
rowCounter = firstRow
lastRow = Range("A65000").End(xlUp).Row
Set FleetData = [A10:S14]

If IsNumeric(xMonths.Value) Then months = xMonths.Value Else
If IsDate(xMonths.Value) Then months = (xMonths.Value - Date) / 30
cost = 0
    Do While rowCounter < lastRow
        Set component = Range(Cells(rowCounter, 1), Cells(rowCounter, 19))
        Set dateOfAction = Cells(rowCounter, 7)
        Set totalApprox = Cells(rowCounter, 12)
        If dateOfAction <= dateHorizon Then
            cost = cost + totalApprox
        End If
        totalCosts = cost
        rowCounter = rowCounter + 1
    Loop

End Function

我使用的数据是：

DateOfAction totalApprox 
5/07/2014    $30,068.62 
24/05/2005   $6,300.00 
5/07/2012    $29,742.00
5/07/2012    $4,360.28
27/12/2012   $5,555.89

单击单元格似乎可以移动值，但没有可识别的顺序。

谷歌搜索并查看了这里，但到目前为止似乎没有什么可以解决问题。

任何帮助将不胜感激！