Block CEO and Twitter co-founder Jack Dorsey has rolled out a new app designed to estimate users’ sun exposure and potential vitamin D synthesis.
Dorsey introduced the app, named Sun Day, to his followers on X on Sunday. According to its white paper on GitHub, Sun Day calculates vitamin D production from UV exposure using a multifactor scientific model. It is currently available for beta testing via Apple’s TestFlight platform, where developers collect feedback before public release.
track your (vitamin) D
a Sun Day app for Sunday.https://t.co/KKsq3LkLEj
— jack (@jack) July 13, 2025
The app relies on location services to monitor the UV index at a user’s current position. To personalise estimates, it also factors in age, skin tone, clothing type, and data from Apple’s Health app, including past vitamin D records.
However, the app does not automatically detect sunlight. Users must manually tap a button when they go outdoors and again when they return inside. Responding to questions on X, Dorsey clarified that Sun Day operates as a manual on/off toggle, logging each exposure session for future reference.
Future upgrades listed in the white paper include incorporating user height, weight, seasonal adjustments, and actual vitamin D blood test results for greater accuracy. The team emphasised that Sun Day aims to deliver conservative, safety-focused estimates tailored to individual profiles.
Sun Day is not Dorsey’s only recent venture. Last weekend, he launched Bitchat, a decentralized peer-to-peer messaging service that operates entirely via Bluetooth. In a post, he shared that his time was spent learning about Bluetooth mesh networks, relays, message encryption models, and related technologies.
When asked if these back-to-back launches were part of a strategy to flood the market with experimental apps, Dorsey replied simply: “Nah. I’m learning.” He also revealed that both apps were built with support from Block’s AI assistant, Goose.
In April 2024, Jack Dorsey announced his departure from the board of Bluesky, the decentralized social media platform he helped create and fund since 2019 and during his tenure as Twitter’s CEO.
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