Sun Day is a new app that helps you measure the vitamin D you intake

Jack Dorsey, co-founder of Twitter and CEO of Block, has unveiled a new application called Sun Day, which aims to provide users with detailed insights into their sun exposure and vitamin D synthesis. Currently available on Apple’s TestFlight platform, the app uses location data to calculate the current UV index and relies on manual user input to log time spent in the sun. By tapping a button when going outdoors and again upon returning inside, users allow the app to measure exposure periods. Sun Day then processes this information alongside additional data points, such as age, clothing type, skin tone, and records from the Apple Health app.

The core objective of Sun Day is to deliver scientifically grounded, personalized estimations of vitamin D production resulting from UV exposure. It does so using a multifactorial model based on medical research. According to the app’s white paper, the methodology prioritizes safety and caution, offering conservative yet reliable recommendations. Future updates are expected to introduce more refined personalization, including calculations based on a user's height and weight, seasonal variations in UV exposure, and potential integration with actual vitamin D blood test results for even greater accuracy.

This latest release is the second app Dorsey has launched within just two weeks. Prior to Sun Day, he introduced Bitchat, a decentralized messaging service that operates over Bluetooth, enabling peer-to-peer communication without relying on centralized servers or internet connectivity. Bitchat is part of Dorsey’s broader exploration of decentralized communication infrastructure, incorporating technologies like Bluetooth mesh networking, relays, and encrypted message forwarding models. Both Sun Day and Bitchat were developed with the assistance of Goose, Block’s open-source AI tool, which Dorsey has been actively using in his recent projects.

These ventures highlight Dorsey’s growing interest in artificial intelligence and decentralized technology—a space he has long supported as a pathway to greater privacy, autonomy, and innovation. His recent work is emblematic of a larger vision that sees AI not simply as a tool for automation, but as a means of creating thoughtful, user-centric digital experiences. Through apps like Sun Day, Dorsey is positioning himself at the intersection of wellness tech and AI innovation, with a focus on offering tangible benefits in users’ everyday lives.

The launch of Sun Day also brings new considerations to the conversation around health tech. As more consumers turn to apps to monitor and manage aspects of their well-being, questions about data privacy and accuracy become increasingly important. Although Sun Day emphasizes user safety and draws from research-backed models, the reliance on self-reported data and mobile sensors will undoubtedly be scrutinized as the app gains traction. Integrating medical-grade data in future updates could help bridge that gap, offering a more comprehensive wellness tool rooted in real-world diagnostics.

Meanwhile, Dorsey’s dual push—on one hand into personal health and wellness, and on the other into decentralized communication—reveals a broader ambition: reshaping the way people interact with technology. Both apps encourage a more conscious engagement with digital tools. Sun Day promotes mindfulness about daily habits and exposure to natural elements, while Bitchat fosters more private, autonomous communication, free from traditional data collection systems.

Beyond their individual utility, these applications signal a philosophical shift in tech development, one that values minimalism, user control, and community-driven evolution. The open-source nature of both projects aligns with this ethos, inviting collaboration and transparency in how the software is built and evolves. By placing the tools and the code in the hands of the public, Dorsey is reinforcing the importance of trust and openness in digital innovation.

As the broader industry continues to grapple with questions of ethics, surveillance, and centralized power, Dorsey’s recent launches offer a glimpse into a possible alternative path—one that embraces decentralization and individual agency. While still in early stages, Sun Day and Bitchat could serve as early indicators of how future tech ecosystems might function: more personal, more private, and more attuned to human needs.

In summary, the release of Sun Day represents Jack Dorsey’s continued commitment to developing meaningful, human-centered technology. By combining scientific research with open-source innovation, the app seeks to empower users with practical tools for monitoring sun exposure and vitamin D intake. Together with Bitchat, it underscores a broader mission to rethink digital interaction in ways that prioritize autonomy, privacy, and real-world benefit. As Dorsey deepens his involvement in AI and decentralized platforms, his contributions are poised to influence not just product design, but the philosophical direction of technology itself.