Twitter co-founder and Block CEO Jack Dorsey impresses with his second app in a week. To test it, you will have to go outside.
The developer presented the beta version of the Sun Day app for iPhone, which is already available through Apple TestFlight. It sounds strange, but its direct duty is to track how much vitamin D you have received from the sun. The app will rely on the UV index, location, and personal data of the user.
Sun Day uses geolocation to find out the current level of ultraviolet radiation in the region. However, there is no fully automatic operation: the user needs to press the button at the start of sun exposure and again when they enter the room. The app also saves data after each session.
Sun Day takes into account age, skin type, clothing, and can also use data from Apple Health, including the history of vitamin D blood tests in HealthKit. In the documentation for GitHub states that it calculates vitamin D synthesis using a multifactorial model based on scientific research. In the future, Jack Dorsey plans to add the ability to make more accurate calculations. In particular, height and weight, seasonal adjustments, and even actual test results.
«The app aims to provide personalized, accurate estimates while remaining conservative for safety», — the official publication states.
track your (vitamin) D
a Sun Day app for Sunday.https://t.co/KKsq3LkLEj
— jack (@jack) July 13, 2025
This is the second app from Dorsey in a week. Last time he introduced Bitchat — a service for messaging via Bluetooth without an Internet connection. But it happened that «safe» Bitchat has not passed the security test and vulnerable to data theft. However, it’s too early to be too harsh, as Dorsey is making both applications as part of a personal experiment. According to him, it’s not an attempt to impose a new trend on the market, but simply to learn. He is learning how to create things quickly with the help of open source and Block’s AI assistant Goose.
But Jack Dorsey was not the first to force users to go outside. TechCrunch saidhow developer Reece Kentish created the Touch Grass app, which locks the phone and forces the user to take a break by touching the grass. This is not a metaphor: the user has to go outside and take a picture of themselves literally touching the grass.
Source: GitHub
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