Now Jack Dorsey has built an app to track your vitamin D

Twitter co-founder Jack Dorsey has released a new app that tracks your sun exposure and vitamin D levels. It’s his second app in a week, after last weekend’s encrypted peer-to-peer messaging app Bitchat.

Sun Day is available now for iOS via TestFlight, and the code for the project is available on GitHub. The app uses location-based data to display your local UV index and sunlight hours. You can detail your skin type and clothing level, then manually toggle when you’re in and out of the sun and the app will track your rough vitamin D levels for the day, along with how long you can be in direct sunlight without burning.

Dorsey says he’s “learning” through his recent weekend coding projects, which he’s developing using the open-source AI coding tool Goose. It and similar vibe coding tools are one of the next key battlegrounds for tech’s AI giants — just last week a $3 billion deal for OpenAI to buy AI coding tool Windsurf fell through, as Google swooped in to hire Windsurf’s CEO and top talent onto its DeepMind team.