By futureTEKnow | Editorial Team
Jack Dorsey, co-founder of Twitter and CEO of Block, has introduced Sun Day, an innovative iOS app designed to help users monitor their sun exposure and estimate vitamin D levels. Currently in beta on Apple’s TestFlight, Sun Day uses scientific models to provide personalized insights based on real-time environmental data and user inputs.
track your (vitamin) D
— jack (@jack) July 13, 2025
a Sun Day app for Sunday.https://t.co/KKsq3LkLEj
Sun Day collects your location to access the local UV index, cloud cover, and sunrise and sunset times. Users enter personal details such as their skin type—categorized into six groups from very fair to very dark—and the type of clothing they wear outdoors. By manually toggling a tracking button when going outside and returning indoors, the app calculates safe sun exposure times before burning and estimates vitamin D production during each session.
The app syncs data with Apple Health, allowing users to consolidate their wellness information. Future updates aim to improve accuracy by incorporating additional personal factors like height, weight, and vitamin D blood test results, making Sun Day a more comprehensive health tool.
Sun Day was developed using Goose, an open-source AI coding assistant created by Block. This reflects Jack Dorsey’s new approach to app development, where AI generates code based on developer input, streamlining the creation process. Sun Day follows closely after Dorsey’s recent launch of Bitchat, a peer-to-peer messaging app, indicating a renewed focus on practical, health-oriented applications.
By merging scientific modeling, real-time environmental data, and AI-assisted coding, Sun Day offers a unique way to manage sun exposure safely while optimizing vitamin D levels. Its simple interface and personalized insights make it a valuable tool for anyone looking to balance sun enjoyment with skin health.
Real-time tracking of UV exposure and vitamin D synthesis
Personalized burn limits based on skin type and clothing
Manual session tracking for accurate data logging
Integration with Apple Health for holistic wellness monitoring
Developed using AI-powered coding assistant Goose
Planned enhancements using height, weight, and blood test data
Sun Day exemplifies how emerging technologies can empower users to take control of their health with precision and ease.
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