To get enough sunlight for your body to produce vitamin D, the conditions need to be just right: the sun needs to be shining somewhat directly down on you. If you live near the equator, that includes the middle of every day. For the rest of us, we get our best vitamin D time in the middle of the day in the summer. But how do you know if you’re really getting enough vitamin D from sunlight? To find out, have a look at this app.