2 .To use the table simply specify the date, time interval, and location then click on the “Compute Table” button. Look at the altitude list down to the 50 range and those are the times that it is safe and beneficial to be exposed to the sun. This is translated to the date and time of places on the globe. It means, for example, that some people who live in Chicago the UVB rays are not potentially present until March 25, and by September 16th it is not possible to produce any vitamin D from the sun in Chicago. Please understand it is only theoretically possible to get UVB rays during those times. If it happens to be cloudy or raining, the clouds will also block the UVB rays.