Trying to update my pivot table with powershell

 I'm trying to automate a Pivot Table update. I put outputs in the code so you can see what I'm trying to update and with
what values. I have a request to have them auto updated so users don't have to change shipdate and do a refresh. Can this be done with a Powershell script? I posted in PS forum and they suggested I try here..

To

 $date = 2015-09-07

From

 $dispshp = [DATE].[Ship Date].&[2015-09-01]

Update:

$workSheet.PivotTables(1).PivotFields("[DATE].[Ship Date]").CurrentPageName=$sdate

#Set the file path (can be a network location)
$filePath ="C:\shipment.xls"#Create the Excel Object
$excelObj =New-Object-ComObjectExcel.Application#Make Excel visible. Set to $false if you want this done in the background
$excelObj.Visible= $true#Open the workbook
$workBook = $excelObj.Workbooks.Open($filePath)#Focus on the top row of the "Data" worksheet#Note: This is only for visibility, it does not affect the data refresh
$workSheet = $workBook.Sheets.Item("Customer")

$sdate="{0:yyyy-MM-dd}"-f (Get-Date)# '2012-08-30

$sdate

$workSheet.Select()

$dispshp = $worksheet.PivotTables(1).PivotFields("[DATE].[Ship Date]").CurrentPageName#$workSheet.PivotTables(1).PivotFields("[DATE].[Ship Date]").CurrentPageName=$sdate

$dispshp

#Refresh all data in this workbook
$workBook.RefreshALL()#Save any changes done by the refresh
$workBook.Save()#Uncomment this line if you want Excel to close on its own
$excelObj.Quit()
 This is the error it throws...

Xcel pops up and display's message:
XML for Analysis Parser
   The restriction value provided by the consumer does not match other restrictions or refers to an unkown object.

 The PS error:

Exception setting "CurrentPageName": "Exception from HRESULT: 0x800A03EC"

At C:\powershell\pivot_table_refresh.ps1:25 char:3
+   $workSheet.PivotTables(1).PivotFields("[DATE].[Ship Date]").CurrentPageName =  ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], SetValueInvocationException
    + FullyQualifiedErrorId : CatchFromBaseAdapterSetValueTI