gistfile1.xml

<?xml version="1.0" ?>
<?qbxml version="5.0" ?>
<QBXML>
  <QBXMLMsgsRq onError="continueOnError">
    <InvoiceQueryRq requestID="1" iterator="{iterator}" iteratorID="{iterator_id}">
      <MaxReturned>50</MaxReturned>
      <ModifiedDateRangeFilter>
        <FromModifiedDate>{from_modified_date}</FromModifiedDate>
        <ToModifiedDate>{to_modified_date}</ToModifiedDate>
      </ModifiedDateRangeFilter>
      <IncludeLineItems>true</IncludeLineItems>
      <IncludeRetElement>TxnID</IncludeRetElement>
      <IncludeRetElement>EditSequence</IncludeRetElement>
      <IncludeRetElement>TxnNumber</IncludeRetElement>
      <IncludeRetElement>CustomerRef</IncludeRetElement>
      <IncludeRetElement>TxnDate</IncludeRetElement>
      <IncludeRetElement>RefNumber</IncludeRetElement>
      <IncludeRetElement>Subtotal</IncludeRetElement>
      <IncludeRetElement>SalesTaxPercentage</IncludeRetElement>
      <IncludeRetElement>SalesTaxTotal</IncludeRetElement>
      <IncludeRetElement>AppliedAmount</IncludeRetElement>
      <IncludeRetElement>BalanceRemaining</IncludeRetElement>
      <IncludeRetElement>InvoiceLineRet</IncludeRetElement>
    </InvoiceQueryRq>
  </QBXMLMsgsRq>
</QBXML>

gistfile2.rbx

def parameterize(xml)
  xml.gsub!('{iterator}', iterator_id.blank? ? 'Start' : 'Continue')
  xml.gsub!('{iterator_id}', iterator_id || '')
  xml.gsub!('{from_modified_date}', ((transactions_last_synced_on || 10.years.ago) - 1.day).to_s(:qb_date))
  xml.gsub!('{to_modified_date}', (Time.now + 1.day).to_s(:qb_date))
  xml
end

gistfile3.rbx

ActiveSupport::CoreExtensions::Time::Conversions::DATE_FORMATS.update(
  :qb_date => '%Y-%m-%d'
)