Update Particular Field when Update (EF Code First)

public class Group
    {
        [Key]
        public int id { get; set; }
        [Required]
        [Display(Name = "Group Name")]
        public string GroupName { get; set; }
        public DateTime CreatedOn { get; set; }
        public DateTime? ModifiedOn { get; set; }
    }

CreatedOn  and ModifiedOn will not show in UI

I have Group Class, when add new record (GroupName and CreatedOn) will Insert Value, But

when edit row GroupName and ModifiedOn will update. CreatedOn will no change