{"Conversion failed when converting date and/or time from character string."}

public void Insert(Employee employee)
        {
            using (SqlConnection con = new SqlConnection(strConnection))
            {
                string sql = "INSERT INTO Employee(EmployeeCode, Firstname, Lastname, Email, Gender ,HireDate, DeptId)";
                sql += " VALUES ('" + employee.EmployeeCode + "', '" + employee.Firstname + "', '" + employee.Lastname + "', '" + employee.Email + "', '" + employee.Gender + "', '" + employee.HireDate + "', '" + employee.DeptId + "')";
                SqlCommand cmd = new SqlCommand(sql, con);
                con.Open();
                cmd.ExecuteNonQuery();
            }
        }

[Table("Employee")]
    public partial class Employee
    {
        [Key]
        public int UserId { get; set; }

        [Required]
        [StringLength(50)]
        public string EmployeeCode { get; set; }

        [Required]
        public string Firstname { get; set; }

        [Required]
        public string Lastname { get; set; }

        [Required]
        public string Email { get; set; }

        [Required]
        [StringLength(10)]
        public string Gender { get; set; }

        [DataType(DataType.Date)]
        [DisplayFormat(DataFormatString = "{0:yyyy-MM-dd}", ApplyFormatInEditMode = true)]
        [Column(TypeName = "date")]
        public DateTime? HireDate { get; set; }

        [Required]
        public int? DeptId { get; set; }

        public int? SectionId { get; set; }

        public virtual Department Departments { get; set; }
    }