Exporting accountExpires

Hi 

i'am using an extension rule for exporting the employeeEndDate  value to accountExpires. 

I have an issue ,when i delete the value in FIM, I can't manage the update of the accountExpires to set it to 0 

 if (mventry[MVConst.empEndDate].IsPresent)
                    {
                        CultureInfo provider = CultureInfo.InvariantCulture;

                        if (mventry[MVConst.empEndDate].Value != null)
                        {
                            DateTime dt = DateTime.ParseExact(mventry[MVConst.empEndDate].Value, "yyyy'-'MM'-'dd'T'HH':'mm':'ss'.000'", provider);
                            csentry["accountExpires"].IntegerValue = dt.AddDays(1).ToFileTime();

                        }
                        else
                        {
                            csentry["accountExpires"].IntegerValue = 0;
                        }


                    }
                    else {
                        csentry["accountExpires"].IntegerValue = 0;
                    }

Any idea