Twitter co-founder Jack Dorsey is on an app-building spree. After releasing a Bluetooth-based messaging app called Bitchat earlier this month, this weekend Dorsey revealed another app called Sun Day that ostensibly helps users track UV exposure and Vitamin D intake.
The new app is available via TestFlight on iOS, and its code is on GitHub for anyone to clone.
Like other developers, Dorsey is riding the vibe-coding bandwagon. But instead of using Cursor, Claude Code, or Windsurf, he says he is using an open-source coding tool called Goose.
Sun Day is simple to use. It shows you the UV Index for your location along with details like cloud cover, sunrise, and sunset timings. You can choose your skin type from six options, and what kind of clothing you’re wearing. Based on these parameters, the app will calculate how long you can expose your skin to sunlight before it starts to burn.
The app also shows your minimum vitamin D intake. Start a session by tapping on the “Track UV Exposure” button, and after the session is ended, the app will show how much vitamin D you’ve gained throughout the session, and in the day.