Jack Dorsey’s new Sun Day app tells you exactly how long to tan before you burn

Twitter cofounder Jack Dorsey is back with a new app that tracks sun exposure and vitamin D levels.

Sun Day uses location-based data to show the current UV index, the day’s high, and additional details like cloud cover, sunrise, and sunset times. Users can choose their skin type from six options and indicate their clothing, such as “light (shorts & t-shirt)”, then manually toggle when they’re in or out of the sun.

Based on this input, the app calculates how long you can safely expose your skin before it begins to burn. That’s an especially helpful feature for those like Dorsey, who mentioned in an X thread that he does not actually use sunscreen.

The app also tracks a user’s estimated vitamin D intake while outdoors. After tapping the “Track UV exposure” button to start a session, the app shows how much vitamin D was absorbed once the session ends. In future updates, additional factors like height, weight, and actual vitamin D blood levels may be included to improve the app’s accuracy.

This marks the second new app Dorsey has launched in a week. But this one may especially appeal to Gen Z, whose tanning obsession has sparked a trend of tracking UV exposure to optimize time spent basking in the sun.

In 2025, the importance of sunscreen isn’t lost on most. Google searches for “best anti-ageing sunscreen” and “UV damage prevention” have more than doubled over the past two years. Yet on TikTok and Instagram, tanning is back in vogue.

Girls flaunt their bikini strap marks, share their tanning routines, and even create sun tattoos (aka skin damage). Some are faking it using blush, bronzer, or spray tans to mimic the sunkissed, or sunburnt, look. Others track the UV index to find the “best” time to tan.

A new survey from the American Academy of Dermatology found that over a fifth of young adults prioritize getting a tan over protecting their skin. One in four say it’s worth looking great now, even if it means looking worse later (just five sunburns more than doubles your risk of developing melanoma).

For those looking to chase (or avoid) the sun, Sun Day isn’t yet available in the App Store. However, a TestFlight beta is open to iOS users, and the code is available on GitHub.