Sun Day is a strange entry in this category, and I mean that approvingly. Most UV apps compete on feature breadth: barcode scanners, wearables, AI skin-age analyzers, mole trackers, community feeds. Sun Day competes by stripping all of that out. One screen. UV index, safe sun time, estimated vitamin D production. Open source code on GitHub if you want to audit the math. It is the most opinionated UV app I have tested, and the opinion is ‘less is more.’
What Sun Day is
Sun Day is a free, open source iOS app from Block Inc. (Jack Dorsey’s company, the parent of Square, Cash App, and Tidal). It launched on TestFlight in July 2025. The app provides safe-sun-time calculations based on current UV index, your skin type, your clothing coverage, and current weather, along with an estimated vitamin D IU per session and a daily history view. The interface is a single screen with minimal chrome. There is no premium tier, no upsell, no community feed, no barcode scanner. The code is available on GitHub for review. The business model is essentially none, which is consistent with how Block treats its open source projects.
Who it’s for
People who value simplicity and auditability over feature breadth. Developers and privacy-conscious users who want to read the source code before installing a UV calculator. iOS users who already have a sun-protection routine and just want a clean single-screen reference for current UV and safe exposure time. Anyone who has tried QSun or other feature-dense UV apps and felt overwhelmed by the UI.
Not the right tool for Android users (iOS-only at the time of testing, and the TestFlight distribution adds friction even for iOS users). Not the right tool if you want reapplication reminders (Sun Day does not push you; it expects you to check the app yourself). Not the right tool if you want a sunscreen barcode scanner, a mole tracker, a wearable, or any of the feature-rich layers other apps offer. Sun Day does not have these and is not adding them.
Features that matter
- Safe-sun-time calculator. Calculates how long you can stay in the current UV conditions, given your skin type and clothing coverage, before burn risk. Similar math to dminder but with a simpler interface.
- Estimated vitamin D IU per session. Tells you roughly how much vitamin D your body can produce during the safe exposure window. Less calibrated than dminder (no body weight input in the version I tested), but directionally useful.
- Daily totals and history. Tracks accumulated UV exposure and estimated D production across the day, with a simple history view.
- Open source. Code on GitHub. You can read exactly how the safe sun time and vitamin D calculations are derived. This is genuinely rare in the consumer health app category and worth flagging.
- Minimalist single-screen UI. No tabs, no menus, no notifications by default. One screen, three numbers, done.
My contrarian take
The interesting question with Sun Day is whether minimalism in a UV app is a virtue or a missed opportunity. The argument for minimalism is real: most UV apps suffer from feature creep, the data they all show is similar, and the differentiation is mostly cosmetic. Stripping the app down to three numbers that actually drive behavior (current UV, safe exposure time, estimated D production) is a respectable design philosophy. The argument against is also real: the most useful feature in UVLens is the reapplication reminder, which Sun Day does not have. The most useful feature in QSun is the barcode scanner, which Sun Day does not have. So you are trading reminders, scanning, and calibration depth for cleaner UI and open source transparency. Worth saying: this is a Block Inc. project. The maintenance story is uncertain. Block has shipped open source projects that thrived and others that quietly died. Bet on the simplicity, not on the long-term roadmap.
Real-world test
I tested Sun Day for 14 days starting in mid-April, mostly during the Lisbon spring UV ramp where index values were climbing from 6 to 9 day over day. I installed via TestFlight (still the distribution channel as of testing). The app launched fast, asked me my skin type and typical clothing coverage on first run, and dropped me onto the main screen. UV index, safe sun time, estimated D production. That is the entire app surface.
The safe sun time numbers were conservative and aligned roughly with what dminder gave me for similar parameters. On a clear UV 8 day with a Fitzpatrick III input and shorts plus t-shirt clothing, Sun Day suggested about 20 minutes of safe exposure. dminder’s calculator gave me a similar number with more inputs. The daily history is simple and functional, just a stack of session estimates. I never opened a settings screen because there was almost nothing to set. The lack of reapplication reminders meant that on day 7, when I was sitting in a park for two hours, I would have appreciated a push notification. UVLens would have given me one. Sun Day did not.
That gap is the whole tradeoff. I found the silence refreshing for about 10 days, then mildly inadequate, then I reinstalled UVLens alongside Sun Day. The combination worked well: Sun Day for the clean reference, UVLens for the reminders.
How it compares
Against UVLens, Sun Day is simpler and open source but lacks reapplication reminders. Against QSun, Sun Day is dramatically lighter but lacks the barcode scanner, the mole tracker, and the wearable option. Against dminder, Sun Day has a cleaner UI but shallower calibration; dminder factors body weight and body-area exposure, Sun Day does not in the version I tested. Against your phone’s built-in weather UV index, Sun Day adds skin-type-aware safe-time calculation, which the weather app does not have. The honest verdict is that Sun Day is the right second app in a UV toolkit, not the only app. Pair it with UVLens for reminders or with dminder for deeper D calibration.
FAQs
Is Sun Day available on Android? Not at the time of testing. iOS only, distributed via TestFlight.
Is it really open source? Yes. The code is on GitHub under a Block Inc. organization. You can read how the safe sun time and vitamin D estimations are calculated.
Why is it on TestFlight instead of the App Store? TestFlight is for beta distribution. Whether Sun Day graduates to the full App Store depends on Block’s roadmap. The TestFlight version is functional and stable in my testing.
Does it have reapplication reminders? No. This is the most-asked-for feature based on user feedback, but the current version does not push notifications. You check the app yourself.
Will Block Inc. keep maintaining this? Uncertain. Block has a mixed track record on open source longevity. Treat it as a useful current tool rather than betting on a long roadmap.
For the rest of the sun and UV apps tested this round, the sun-uv-tools hub covers the protection-focused and optimization-focused options. Sun Day is the cleanest minimal entry in the category. If you find yourself buried by feature-rich apps and want a single-screen reference, install it.