If you want to show invoice date in sales order form then you need to add field invoicedate in salestable and update date using following code.
CustInvoiceJour custinvoiceJour1;
SalesTable SalesTable1;
;
while select forupdate * from SalesTable1 where SalesTable1.InvoiceId=="" && SalesTable1.SalesStatus==SalesStatus::Invoiced
{
select custInvoiceJour1 order by invoicedate desc where custInvoiceJour1.SalesId == SalesTable1.SalesId && custInvoiceJour1.SalesId != "";
ttsbegin;
SalesTable1.InvoiceDate=custInvoiceJour1.InvoiceDate;
SalesTable1.update();
ttscommit;
}