On the heels of last weekend’s surprise launch of Bitchat, an encrypted peer‑to‑peer messaging service that runs over Bluetooth, Twitter co‑founder Jack Dorsey quietly dropped another beta: Sun Day, a minimalist iOS app designed to help you quantify your sun exposure—and by extension, your vitamin D intake. Available now via TestFlight, Sun Day is open‑source, its code sitting on GitHub for anyone curious enough to poke around.

Dorsey describes these projects as part of his personal learning curve—“vibe coding,” as he calls it—tackling one new idea each weekend and building it using Goose, an open‑source AI coding assistant developed by his payments company, Block. In just days, he’s dipped into Bluetooth mesh networks and message‑encryption models for Bitchat, then pivoted to UV indices and health‑tech for Sun Day. The code‑first approach underscores Dorsey’s continued faith in open‑source principles: he not only uses Goose but shares every line of code for public inspection and collaboration.

How Sun Day works

At its core, Sun Day is astonishingly simple—by design. After granting location access, the app:

1. Pulls your local UV Index and sunlight hours from public meteorological data.
2. Asks you to specify your skin type (six categories, from very fair to very dark) and clothing coverage (e.g., shorts and T‑shirt vs. long sleeves and pants).
3. Logs sun sessions via a manual on/off toggle—tap “in” when you walk into sunlight, “out” once you head back indoors.
4. Estimates vitamin D synthesis using a multifactor model detailed in its white paper on GitHub, based on peer‑reviewed research into UV‑driven cholecalciferol production.

The outcome: a daily gauge of roughly how much vitamin D you’ve generated—plus a conservative “safe‑exposure” countdown that warns when you’re nearing potential sunburn.

Vitamin D deficiency affects an estimated one billion people worldwide, contributing to bone‑density loss, immune dysregulation, and even mood disorders like seasonal affective disorder (SAD). While nutritionists caution that sun exposure carries skin‑cancer risks, moderate UVB is the most efficient way for most people to top up their vitamin D stores. By blending real‑time UV indices with personal factors, Sun Day aims to empower users to balance the benefits of sunshine against its hazards.

• Sunlight vs. supplements: Unlike generic supplement reminders, Sun Day offers a dynamic, location‑aware alternative—tailoring guidance to today’s cloud cover, your latitude, and even seasonal shifts.
• Manual tracking tradeoffs: Dorsey’s choice for manual toggles over continuous light‑sensor monitoring is deliberate: “It doesn’t look for light,” he explained on X, “it’s a manual on/off toggle”—prioritizing user control and privacy over automated tracking.

Sun Day’s rapid turnaround spotlights a wider contest among AI‑driven developer tools. Just last week, OpenAI’s proposed $3 billion acquisition of Windsurf, a code‑generation startup, fell through when Google scooped up Windsurf’s CEO and core team for its DeepMind division. Meanwhile, Dorsey is backing Goose, betting that open‑source AI assistants—rather than closed‑source models—will democratize coding and spark grassroots innovation.

By pushing Sun Day into TestFlight and GitHub, Dorsey invites feedback on everything from its UV algorithms to UI polish. Future updates flagged in the white paper include:

• More granular inputs (height, weight, blood‑test integration via HealthKit)
• Seasonal adjustments for solar angle and atmospheric conditions
• Machine‑learning smoothing to refine burn‑risk warnings based on user history

Whether Sun Day evolves into a widely adopted health widget or remains a weekend experiment depends on community uptake—and whether users find manual toggles intuitive enough to sustain daily use.

If you’re outdoorsy, health‑curious, or simply chasing the next cutting‑edge app, Sun Day is worth a weekend download:

• Test it now (iOS via TestFlight) to see your local UV index, sun‑hour predictions, and how small tweaks in clothing or timing could boost—or bust—your vitamin D goal.
• Inspect the code on GitHub to suggest improvements, add features, or fork your own version.
• Join the conversation: On X, Dorsey has been transparent about the app’s limits and ambitions—this is as much a community project as a solo sprint.