Twitter co-founder Jack Dorsey has unveiled his second mobile app in the past few weeks. His new project, called Sun Day, is designed to help users track their sun exposure and vitamin D intake, TechCrunch reports. Just last week, Dorsey launched Bitchat, a Bluetooth-based instant messaging app for local messaging.
Sun Day is currently available via TestFlight for iOS, and its source code is published on GitHub, so anyone can create their own version.
The app provides users with the UV Index for their location, as well as weather data such as cloud cover, sunrise and sunset times. Users can select their skin type (one of six) and what clothing they are wearing. Based on these parameters, the app calculates a safe amount of time to stay in the sun before getting burned.
Sun Day also estimates your body's minimum requirement for vitamin D. A session starts after you tap the Track UV Exposure button. Once completed, the app will show you how much vitamin D you absorbed during that session and for the day as a whole.
Unlike other developers who are increasingly using AI assistants like Cursor, Claude Code, or Windsurf, Dorsey used the open-source programming tool Goose to create Sun Day.
Dorsey's new app is designed to tap into the "sun-conscious living" trend that is gaining popularity among both the tech community and medical experts concerned about the effects of both vitamin D deficiency and excessive exposure to ultraviolet light.