Add SPitem issue

Requirement: create SPlistitem for each submission date and start date for a customer. Submission and start dates are managed metadata fields and allows multiple values.

Code below updates submission date (from deadlinecollection) Okay but for the startbycollection it only sets the field for the last item in start date.

   foreach (TaxonomyFieldValue deadlineitem in deadlinecollection)

                       {

                           SPListItem submission = ilsubmissions.Add();

                          Submission ["Customer Number"] = ilreturn["CustomerNumber"].ToString();

                          foreach (TaxonomyFieldValue startbyitem in startbycollection)

                           {

                               string sdate = startbyitem.Label + "/" + returnyear;

                               DateTime startdate = Convert.ToDateTime(sdate);

                               Submission ["Start By"] = startdate;

                               submission.Update();

                           }

                           string date = deadlineitem.Label + "/" + returnyear;

                           DateTime returndate = Convert.ToDateTime(date);

                           submission["Submission Date"] = returndate;

                           submission.Update();

                       }

What am I doing wrong here?

nish