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.
track your (vitamin) D
a Sun Day app for Sunday.https://t.co/KKsq3LkLEj — jack (@jack) July 13, 2025
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.
Image Credit: TechCrunch
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.
TechCrunch News: Jack Dorsey is experimenting with apps to monitor sunlight exposure and vitamin D.
Sun, code, and vitamin D! Jack Dorsey amazes the world once again with his new app, Sun Day—it measures the UV index and calculates safe time in the sun.
TechCrunch новость: Джек Дорси экспериментирует с приложениями для контроля солнечного воздействия и витамина D.
Солнце, код и витамин D! Jack Dorsey снова удивляет мир: его новое приложение Sun Day измеряет UV-индекс и рассчитывает безопасное время пребывания на солнце.
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