There is an app I've been using to track my Vitamin D for a few years now: http://dminder.ontometrics.com
(Not affiliated with the app, just happy with it.) It tells you the sun angle, peak hour of the day, and maximum time you should be in that day's sun based on your skin type, to avoid getting burnt while still getting enough D.
Be careful though - you might get a little OCD about Vitamin D tracking with this app. I did, and mainly use it now for checking how long it is safe to stay outdoors. (Instead of full blown tracking.)