How can i update Datetime

MODEL

`public class TDORDERS : INotifyPropertyChanged

{

    public event PropertyChangedEventHandler PropertyChanged;

    [PrimaryKey, AutoIncrement]

    public int TDId { get; set; }

    public DateTime? _tddate { get; set; }

public DateTime? TD_DATE

    {

        get { return _tddate; }

        set

        {

            if (_tddate == value)

                return;

            _tddate = value;

            OnPropertyChanged();

        }

    }

private void OnPropertyChanged([CallerMemberName] string propertyName = null)

    {

        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

    }

}`

** How can i update TD_DATE with current datettime corresponding to TDId **