更改c# Xamarin中的日期格式

我在Android上工作Xamarin。我在我的项目中使用了DatePicker。代码如下:

OnCreate方法内部代码。

protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            SetContentView(Resource.Layout.Activity_register);
            tiebirthdate = FindViewById<TextInputEditText>(Resource.Id.tiebirthdate);
            tiebirthdate.Click += DateSelect_OnClick;
       }

void DateSelect_OnClick(object sender, EventArgs e)
        {
            DatePickerFragment frag = DatePickerFragment.NewInstance(delegate (DateTime time)
            {
                var dateAndTime = time;
                var tt = dateAndTime.Date;
                tiebirthdate.Text = tt.ToString("dd/MM/yyyy");
            });
            frag.Show(FragmentManager, DatePickerFragment.TAG);
        }

创建类。

class DatePickerFragment : DialogFragment,
                                  DatePickerDialog.IOnDateSetListener
    {
        public static readonly string TAG = "X:" + typeof(DatePickerFragment).Name.ToUpper();
        // Initialize this value to prevent NullReferenceExceptions.
        Action<DateTime> _dateSelectedHandler = delegate { };
        public static DatePickerFragment NewInstance(Action<DateTime> onDateSelected)
        {
            DatePickerFragment frag = new DatePickerFragment();
            frag._dateSelectedHandler = onDateSelected;
            return frag;
        }
        public override Dialog OnCreateDialog(Bundle savedInstanceState)
        {
            DateTime currently = DateTime.Now;
            DatePickerDialog dialog = new DatePickerDialog(Activity,
                                                           this,
                                                           currently.Year,
                                                           currently.Month,
                                                           currently.Day);
            return dialog;
        }
        public void OnDateSet(DatePicker view, int year, int monthOfYear, int dayOfMonth)
        {
            DateTime selectedDate = new DateTime(year, monthOfYear + 1, dayOfMonth);
            _dateSelectedHandler(selectedDate);
        }
    }

我尝试后，最后我设置DateTime像这样的格式"dd/MM/yyyy"。但是我的服务器需要像"yyyy-MM-dd'T'HH:mm:ss'Z'"这样的日期格式。那么如何更改DateTime格式，如upper.

tiebirthdate.Text = dateAndTime.ToString("yyyy-MM-dd'T'HH:mm:ss'Z'", CultureInfo.InvariantCulture);