Jack Dorsey unveils Sun Day

Jack Dorsey, known globally as the co-founder of Twitter and CEO of Block, has been on a creative tear lately, launching back-to-back experimental apps. The latest addition to his portfolio is Sun Day, a simple yet intriguing app aimed at helping users track their sun exposure, monitor UV levels, and estimate vitamin D synthesis, all from their smartphones.

Available via TestFlight for iOS users, Sun Day offers an open-source approach, with its codebase published on GitHub for developers and curious users alike to inspect or remix. This move aligns with Dorsey’s increasing embrace of open-source tools and decentralized tech, as seen in his other recent project, the Bluetooth-based messaging app Bitchat.

Jack Dorsey unveils Sun Day: A minimalist app for sunlight tracking and vitamin D awareness: What does Sun Day actually do?

At its core, Sun Day delivers real-time insights into UV intensity and personal sun exposure limits. The app taps into your location to display the local UV Index, as well as sunrise and sunset times and cloud cover conditions. But it doesn’t stop at weather metrics.

One of its standout features is the personalization layer. Users can select their skin type from six categories (based on the Fitzpatrick scale) and indicate the type of clothing they’re wearing. With this data, Sun Day estimates how long you can safely remain under the sun before risking sunburn.

Once you start a UV tracking session, the app logs your exposure and, at the end, reports how much vitamin D your body has likely produced. This approach reflects an increasing public interest in not just avoiding sun damage but also ensuring sufficient vitamin D levels, especially given global concerns about deficiencies.

The appeal of Sun Day

While the concept of a UV tracker isn’t entirely new, Sun Day leans into a minimalist and transparent ethos. Unlike commercial wellness apps packed with ads, paywalls, or data collection practices, Sun Day is part of an open-source ecosystem. Dorsey wrote the app using Goose, an open-source coding tool, distancing himself from mainstream AI coding assistants like Cursor or Claude Code.

This decision highlights what some in the tech community call “vibe-coding”, an experimental, often fast-moving development culture where indie developers and tech veterans alike spin up tools to solve small, focused problems or just for the joy of building.

For Dorsey, this marks a creative phase beyond big-company leadership. His recent app releases feel less like major product launches and more like explorations, some successful, some controversial.

You can try out Sun Day by downloading Sun Day in test flight.

Developers and curious users can view or clone the code from the official GitHub repository.

Lessons from Bitchat: Why openness needs guardrails

Dorsey’s simultaneous work on Bitchat offers context for Sun Day. Marketed as a secure, decentralized messaging tool using Bluetooth and end-to-end encryption, Bitchat promised private communication without reliance on internet infrastructure.

However, within days of its release, security researchers uncovered multiple vulnerabilities, from broken identity verification to the potential for impersonation attacks. Even Dorsey himself added warnings to the app’s GitHub page, urging users not to trust it for sensitive communication until proper security audits are conducted.

This episode serves as a cautionary tale: openness and transparency are valuable, but they don’t replace the need for rigorous testing, especially when apps touch on health, privacy, or security concerns.

With Sun Day, the stakes are arguably lower, no one’s digital identity is at risk from faulty UV readings, but the pattern raises questions about how indie developers balance rapid iteration with user trust.

Why it matters: Tech’s shift toward DIY wellness

Sun Day lands in a broader wave of wellness tech that focuses on self-quantification and DIY health monitoring. From step counters to sleep trackers to heart rate monitors, consumers today want to understand their bodies better, often outside of clinical settings.

Vitamin D is an especially ripe topic. Studies have shown widespread deficiencies linked to lifestyle changes, indoor work, sunscreen use, and geographic factors. While a smartphone app can’t replace blood tests or professional advice, it can help users become more mindful of their habits and environmental factors.

Looking ahead: Experimental apps as a trend

For developers, Sun Day offers inspiration on several levels:

• Open Source as Invitation: By publishing the code, Dorsey invites remixing, adaptation, and learning, not just passive consumption.
• Simple UX: The app focuses on one core problem (sun exposure tracking) without layering in unnecessary features.
• Rapid Experimentation: Tools like Goose allow developers to prototype quickly, test ideas, and ship, even if the result is imperfect or short-lived.
   

For users, it’s a reminder that tech’s next wave may not come from massive platforms but from small, purpose-built tools emerging from hacker culture and indie labs. Sun Day is not a revolution in wellness tech, but it’s a fascinating glimpse into where Jack Dorsey’s mind is post-Twitter. It embodies a playful, iterative, and open-source spirit, blending tech curiosity with real-world usefulness. Whether it will catch on or fade into the long list of indie experiments, one thing is clear: Dorsey isn’t done building, and his next app may be just as unexpected. For app developers, Sun Day is both a case study and a challenge: What small but meaningful problem could you solve next?