String was not recognized as a valid DateTime. error in sql query

sqlstr = sqlstr + " UPDATE EMP_PER set FNAME='"+txtfname.Text.Trim().ToUpper() + "'";
                sqlstr = sqlstr + " LNAME='" + txtsname.Text.Trim().ToUpper() + "',DOB='" + String.Format("{0:MM/dd/yyyy}",Convert.ToDateTime(txtdob.Text)) + "',Address='" + txtadd.Text.Trim() + "', ";
                sqlstr = sqlstr + " Gender='"+drpgen.SelectedValue + "',Phone='"+txtphone.Text.Trim() + "',";
                sqlstr = sqlstr + " Mobile='"+txtmobile.Text.Trim() + "',Email='"+txtemail.Text.Trim() + "', ";
                sqlstr = sqlstr + " Emergency_Contact='"+txtecontact.Text.Trim() + "',M_Status='"+drpmstatus.SelectedValue + "',";
                sqlstr = sqlstr + " Blood_Grp='" + drpbgrp.SelectedValue + "', PIssuedat='" + String.Format("{0:MM/dd/yyyy}", Convert.ToDateTime(txtpidt.Text)) + "'";

this  is a part of update query in which query gets formed only till M_Status='"+drpmstatus.SelectedValue + "'," and tehn cuts off throwing error



 query generated 

UPDATE EMP_PERSONAL set FNAME='SMITA', LNAME='SHINDE',DOB='05/18/1910',Address='mmmmm',  Gender='M',Phone='', Mobile='',Email='',  Emergency_Contact='',M_Status='M',"

error


String was not recognized as a valid DateTime.