Ok, let's try this workaround, capturing the working string and do a replace on it
Sub myName()
Dim pvtTable As PivotTable
Dim SQLString As String
Set pvtTable = ActiveSheet.PivotTables(1)
With pvtTable.PivotCache
SQLString = .CommandText 'the SQL Query
SQLString = Replace(SQLString, "2008-01-01", "2010-01-01")
.CommandText = SQLString
End With
End Sub