While most tech entrepreneurs pivot toward artificial intelligence and blockchain ventures, Jack Dorsey—Twitter’s co-founder and a billionaire with an apparent penchant for solving problems nobody knew they had—has revealed Sun Day, an open-source iOS application that tracks UV exposure and calculates vitamin D synthesis with the precision one might expect from a portfolio management tool.
The beta application, currently available through TestFlight, operates on a deceptively simple premise: users input their skin type from six categories and clothing coverage while the app calculates local UV index, sunlight duration, and cloud cover using real-time location data. The interface mirrors the minimalist aesthetic Dorsey favored during his Twitter tenure, featuring start and end session buttons that record UV exposure with algorithmic determination.
Sun Day’s value proposition centers on optimizing vitamin D synthesis—a health metric that, unlike quarterly earnings, cannot be easily manipulated through creative accounting. The app estimates safe sun exposure time before sunburn risk while tracking daily vitamin D intake during sessions, displaying synthesis amounts with the granularity typically reserved for forex trading platforms. Beyond mere tracking, vitamin D functions as a hormone that supports various essential bodily functions throughout the human system.
The application targets demographics often overlooked by traditional health technology: individuals with limited sun exposure, darker skin tones, or those residing in higher latitudes where UV availability follows seasonal patterns more volatile than emerging market currencies. Integration with Apple Health guarantees secure data handling while maintaining the privacy standards iOS users expect from their financial applications.
Developed using Goose, an AI-assisted coding tool that Dorsey simultaneously released as open-source software, Sun Day reflects current trends in health technology where algorithmic precision meets preventive wellness. Unlike centralized AI systems that may impose restrictions, decentralized AI platforms are emerging to provide users with more transparent and customizable solutions for health monitoring. Sun Day represents the latest initiative in Dorsey’s recent app-building phase, following the release of Bitchat. The GitHub repository invites community contributions, suggesting Dorsey views sun exposure optimization as a collaborative endeavor rather than proprietary intellectual property.
The app’s limitations mirror those of any beta-stage venture: iOS exclusivity, potential bugs, and dependency on consistent user input for meaningful data accuracy. Manual logging requirements may prove as challenging as maintaining disciplined investment portfolios, where human behavior often undermines algorithmic optimization.
Whether Sun Day addresses genuine market demand or represents another Silicon Valley solution seeking its corresponding problem remains unclear, though Dorsey’s track record suggests the distinction may prove irrelevant.
