Label that renders a countdown timer

CountdownLabel.xaml.cs

using System;
using Xamarin.Forms;

namespace DK.InstantBetting.Views
{
    public partial class CountdownLabel : Label
    {
        public static readonly BindableProperty TargetTimeProperty =
            BindableProperty.Create("TargetTime", typeof(DateTime), typeof(CountdownLabel), DateTime.Now, propertyChanged: TargetTimeChanged);

        public static readonly BindableProperty PrefixProperty =
            BindableProperty.Create("Prefix", typeof(string), typeof(CountdownLabel), string.Empty);

        public DateTime TargetTime
        {
            get => (DateTime)GetValue(TargetTimeProperty);
            set => SetValue(TargetTimeProperty, value);
        }

        public string Prefix
        {
            get => (string)GetValue(PrefixProperty);
            set => SetValue(PrefixProperty, value);
        }

        private bool _countdownRunning = false;
        private bool _countdownValid = true;
        private bool ShouldCountDown
        {
            get => _countdownValid && TargetTime > DateTime.Now;
        }

        public CountdownLabel()
        {
            InitializeComponent();
            UpdateTitle();
            StartCountdown();
        }

        ~CountdownLabel()
        {
            StopCountdown();
        }

        private static void TargetTimeChanged(BindableObject bindable, object oldValue, object newValue)
        {
            CountdownLabel countdownLabel = bindable as CountdownLabel;
            if (countdownLabel == null)
            {
                return;
            }

            countdownLabel.UpdateTitle();
            countdownLabel.StartCountdown();
        }

        public void StartCountdown()
        {
            _countdownValid = true;
            if (_countdownRunning || !ShouldCountDown)
            {
                return;
            }

            _countdownRunning = true;
            Device.StartTimer(TimeSpan.FromSeconds(1), () =>
            {
                if (ShouldCountDown)
                {
                    UpdateTitle();
                }
                _countdownRunning = ShouldCountDown;
                return ShouldCountDown;
            });
        }

        public void StopCountdown()
        {
            _countdownValid = false;
        }

        private void UpdateTitle()
        {
            Text = ShouldCountDown ?
                FormatTimeSpan(TargetTime.Subtract(DateTime.Now)) :
                "";
        }

        private const string DaysFormat = @"dd\:hh\:mm\:ss";
        private const string HoursFormat = @"hh\:mm\:ss";
        private const string MinutesFormat = @"mm\:ss";
        private const string SecondsFormat = @"ss";

        private string FormatTimeSpan(TimeSpan span)
        {
            string format;
            if (span.Days > 0)
            {
                format = DaysFormat;
            }
            else if (span.Hours > 0)
            {
                format = HoursFormat;
            }
            else if (span.Minutes > 0)
            {
                format = MinutesFormat;
            }
            else
            {
                format = SecondsFormat;
            }

            return string.Format("{0}{1}", Prefix, span.ToString(format));
        }
    }
}