How Can Use Non-Stop Timer In All pages?

Hello

I just start learing Xamarin for a few week,now i want a Timer that Works Non-Stop when a i go to another pages! i use MVVM for that, but when i go to another pages and come back it's start from zero again. how can i do that in xamarin?

`           Timer tmr = new Timer();
            tmr.Interval = 1000;
            tmr.Elapsed += delegate
            {
                Sec++;
            };
            tmr.Start();
        }
        private int sec=1;

    public int Sec
    {
        get { return sec; }
        set
        {
            sec = value;
            PropertyChanged(this, new PropertyChangedEventArgs(nameof(Sec)));
        }
    }

`
and in Xaml i use Bindindcontext,it works fine

• even i use this type of code in CS code behind page thats work fine but every time came to this page its start from zero:

Timer tmr = new Timer();
tmr.Interval = 1000;
tmr.Elapsed += delegate {
Sec++;
Device.BeginInvokeOnMainThread(() =>
{
txtsec.Text = Sec.ToString();
});
};
tmr.Start();