Your body only generates D from the UVB rays, which only enter the atmosphere when the sun is at least 30º off the horizon. If you live in a place that is more than 35º from the equator, that also means you will have parts of the year where there is no ability to make D at all, even at noon. The app tells you when you will next be able to make D whether it's an hour from now, or 3 months. On any given day, it will tell you the start and end times for that day (the time when the sun moves higher than 30º and then lower again).

The time of the day that the sun reaches its maximum elevation angle: its highest point in the sky. This is the time of maximum Vitamin D production because the most UVB is entering the atmosphere (assuming the sun is greater than 30º off the horizon that day).

To get Vitamin D, you should go when you will make it at the highest rate, that way you will not have to stay out as long. If you burn easily, you also have a genetic advantage: you make Vitamin D up to 10x faster than people with darker skin. The app considers all these factors. And it will tell you what a safe amount of time to be out is and warn you when you have come up to that amount of time. Your body issues a warning of its own: before you burn, your skin will get pink and/or show patterning. If you abide by the warning times, that should never happen.

Yes. A person who is 50 needs 2x longer than a 20 year old to generate the same amount of D. The application applies this factor in computing the generation rate.

Yes. When using the app, you can just swipe the timer dial side-to-side and the application will ask if you are going to turn over. If you say yes, it will reset the warning timer.

The app will let you set your own target, which it will then use to show you how your performance has been vs. your goal. We recently introduced autopilot to the iOS version: you set the level you want to get to and the app tells you how much you need to get. The RDA recommendation of 600 IUs was chosen because the goal is to be at least 20. Many other experts believe you should shoot for a level more like 40 or 50. Please see the Estimation page.

The free version is fully functional, so you can try it and make sure it works for you. The paid version retains history and continuously estimates your D level. The number one goal of the app is to give you a tool to avoid falling into deficiency. Of course, testing is an important part of that strategy, but testing once a year is insufficient as your D levels can move around quite a bit. Even testing 3x per year is not a guarantee that your levels never got dangerously low (and, of course, a test is after the fact). While it is true that everyone should attempt to achieve optimal levels, avoiding severely deficient ones is by far more important and continuous estimation helps you do that.

Yes. We just recently introduced a version that allows you to put in labs every time you get them and the program will then reset the continuous estimation to begin from that known point forward.

Well... No. First off, UVA rays are the ones that cause photo aging, which is another reason to try to get out around Solar Noon (when the ratio of A to B is optimal). But also, in a not totally surprise irony of nature: sun avoidance actually increases cancer risk. It's believed that this is the case because exposure prompts the body to generate burn defenses, so extensive avoidance makes burns more likely (also, D prevents cancer). After years of telling people to avoid the sun, even dermatologists have backed off that position and are instead recommending safe sun exposure.

Not at all. One of the main uses of the app is to make it so you can go get your D and then either go inside, apply some sun screen, or cover up.