No System.Timers? What's best way to update UI (like chronograph)?

Hi Xamarin Experts,
I tried to use a System.Timers.Timer - but saw that you need to downgrade to 4.0 or go to 4.5.1 to get these? I'm not even able to change to 4.5.1 for some reason.

What's my best option to do this without having to change default project settings?

Thanks!
Mike