NullableDatePicker.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;

namespace BSILabs.Controls
{
    public class NullableDatePicker : DatePicker
    {
        public static readonly BindableProperty NullableDateProperty = BindableProperty.Create(nameof(NullableDate), typeof(DateTime?), typeof(NullableDatePicker), null, BindingMode.TwoWay, propertyChanged: NullableDateChanged);

        public DateTime? NullableDate
        {
            get { return (DateTime?)GetValue(NullableDateProperty); }
            set { SetValue(NullableDateProperty, value); }
        }

        public static readonly BindableProperty NullTextProperty = BindableProperty.Create(nameof(NullText), typeof(string), typeof(NullableDatePicker), default(string), BindingMode.TwoWay);

        public string NullText
        {
            get { return (string)GetValue(NullTextProperty); }
            set { SetValue(NullTextProperty, value); }
        }

        public NullableDatePicker()
        {
            DateSelected += NullableDatePicker_DateSelected;
            NullText = "Select Date...";
        }

        private void NullableDatePicker_DateSelected(object sender, DateChangedEventArgs e)
        {
            NullableDate = new DateTime(
               e.NewDate.Year,
               e.NewDate.Month,
               e.NewDate.Day,
               NullableDate?.Hour ?? 0,
               NullableDate?.Minute ?? 0,
               NullableDate?.Second ?? 0);
        }

        private static void NullableDateChanged(BindableObject obj, object oldValue, object newValue)
        {
            var customDatePicker = obj as NullableDatePicker;
            var newDate = newValue as DateTime?;

            if (customDatePicker != null && newDate.HasValue)
            {
                customDatePicker.Date = newDate.Value;
            }
        }
    }
}