Can AI build a fasting tracker app like Zero?

Build a personal fasting tracker web app to replace Zero Plus for my own use. I only need the timer, streak, and protocol switching, not food logging or wearable sync.

Tech rules:
- Plain HTML, CSS, and vanilla JavaScript in one file, plus a small service-worker.js so it opens offline. No framework, build step, account, or server.
- Store all data in localStorage under one key, with a version number.

Data model:
- Fast: id, startAt, endAt (null while active), plannedHours, protocolId, notes.
- Protocol: id, name, fastingHours, eatingHours (16:8, 14:10, OMAD, 5:2, custom).
- StreakState: currentStreak, longestStreak, lastFastEndAt.
- LogEntry: fastId, type (water, black coffee), timestamp.

Features:
- Start a fast by picking a protocol. Show a live countdown to the planned end time and elapsed time past it.
- End a fast manually at any time, not just at the planned hour.
- Streak counter that increments per completed fast and does not reset just because more than 24 hours passed between two fasts, only when a fast is skipped entirely for a full day.
- Backfill screen to add a past fast with a start and end time after the fact.
- Log water or black coffee during an active fast without ending it.
- Edit or delete any past fast entry.
- Switch between saved custom protocols (16:8, OMAD, 5:2, and a custom hour count).

Edge cases the build must handle correctly:
1. A fast that crosses midnight displays correctly and doesn't split into two entries.
2. A timezone change mid-fast (travel) doesn't corrupt the elapsed-time calculation; store all times in UTC and convert for display.
3. Ending a fast early and starting a new one the same day doesn't break the streak.
4. Logging water or black coffee during a fast does not end or pause the fast.
5. If more than 24 hours pass with no active or logged fast, the streak resets to zero on the next fast start, not silently in the background.
6. Backfilling a fast recalculates the streak as if it had been logged live.
7. Editing a past fast's start or end time recalculates its duration and any streak effect.
8. Deleting a past fast recalculates the streak from the remaining entries.

Backup:
- One-click "Export JSON" and "Export CSV" of all fasts.
- "Import JSON" restores after confirmation.

Layout:
- Mobile-first, large countdown display readable at 375px wide, high contrast.

Exclusions:
- No publishing, App Store submission, accounts, payments, social features, photo food logging, or wearable/Health app sync.

Write a short README explaining where data is stored, how to export and restore it, and how to install the page to a phone home screen so notifications and offline loading work.

Keep it under 500 lines. This is a personal tool, not a product.

What is the short answer?

Yes, but. AI can build a fasting timer with a streak counter and custom protocols to replace Zero Plus at $9.99 a month or $69.99 a year in a few hours.

The “but” is that you’re not actually replacing much. The two things people use daily, a countdown timer and a streak counter, are already on Zero’s free tier along with common presets like 16:8. The $9.99 a month buys preset hour-labels like “Fat-Burning Mode,” custom fast presets layered on that same timer, AI photo food logging, and wearable sync. Build your own if you want protocols or backfill rules Zero doesn’t offer, or if you just don’t want the upsell nudges toward Plus. Don’t build it expecting to save money you’re not currently spending.

Option	Best for	Upfront cost	Ongoing cost	Main limitation
Build a rough prototype	Someone who wants a custom timer and streak without Zero’s app at all	$0 and 2–3 hours	$0	No reliable notifications yet, no health app sync
Build a daily-use personal version	Someone who wants their own protocols, backfill, and edit controls	$0 and 6–9 hours	$0	Still no Apple Health, Fitbit, or Oura sync
Keep paying for Zero Plus	Someone who wants AI photo food logging, wearable sync, or stage-view labels	$0	$9.99/mo or $69.99/yr	Paying for features layered on a timer that’s already free

The ready-to-paste starter prompt at the end of this page builds the daily-use version.

What should the first version include?

The first version needs to survive a real week of fasting, including the days you forget to open the app.

• A live countdown from a chosen protocol
• A streak counter that doesn’t punish a normal missed log
• Multiple protocols (16:8, OMAD, 5:2, custom hours)
• Backfill for a fast you forgot to log
• Water and black coffee logging that doesn’t end the fast
• Deliberate exclusion: no AI photo meal logging, no wearable sync. Those need real infrastructure a personal build shouldn’t attempt.

What can AI build reliably?

ChatGPT and Claude build a fasting timer and streak counter reliably. It’s a countdown, a state machine, and some date math, and there are thousands of similar timer apps to draw on.

Concrete things AI gets right:

• A countdown that correctly shows elapsed time past the planned end
• Multiple named protocols saved and switchable
• A streak counter with sane rules for gaps
• Backfilling a past fast and recalculating the streak

Where will AI need human help?

The edge cases around fasting math need your testing. Fasts that cross midnight, timezone changes mid-fast during travel, and what counts as breaking a fast versus just logging water all need explicit rules, and AI’s first pass often gets the streak logic wrong when more than 24 hours passes between fasts. The starter prompt spells these out, but you should test each one by hand: start a fast, change your phone’s timezone, and confirm the countdown still makes sense.

Editing and deleting past fast entries needs care too. A backfilled or corrected fast should recalculate your streak, not just sit there disconnected from it.

The decision only you can make is whether notifications matter enough to install the page as a home-screen PWA. Reliable fast-start and fast-end push notifications are hard on a plain web page, especially on iOS Safari, so without that install step you’ll be checking the timer manually.

Privacy is low-stakes here. It’s fasting times, not medical data, and it stays on your device with no account.

Can I get my data out of Zero?

Yes, with some friction. Zero’s Settings has a “Download My Data” action that emails you a password-protected link to a zip file. Whether this works on the free tier or requires Plus is inconsistent across sources [VERIFY].

The export trap: the zip contains separate JSON files, not a CSV or spreadsheet-ready table. A third-party open-source Python package exists specifically to parse Zero’s JSON export, which itself signals the raw export isn’t easy to work with. Third-party reviews also describe streak and milestone counters failing to increment correctly, with no in-app way to edit or delete a wrongly-logged fast [VERIFY, from a third-party review, not Zero’s own docs].

The practical path: request the export, unzip it, and either write a short script to convert the JSON fast records into your new app’s format, or just re-enter your current streak count and move forward from today. For most people the second option is faster and the data lost is only historical, not functional.

How long will it take and what will it cost?

Rough prototype: 2–3 hours and $0. A working countdown timer, protocol switching, and a basic streak counter.

Dependable daily-use version: 6–9 hours, including backfill, edit and delete, timezone handling, and installing the page as a home-screen PWA for notifications. Build it on a free AI plan.

There’s no clean break-even here because the timer and streak you’d build are already free in Zero. The math only makes sense if you value custom protocols or want out of the upsell nudges toward Plus, not if you’re trying to save $9.99 a month.

These are estimates from reviewing the feature set, not measurements from a completed build.

Which AI tool or approach should I use?

Claude or ChatGPT on a free plan handles this build. The core timer and streak logic fits in one conversation.

Cursor is unnecessary. The app is small enough to iterate on in chat.

Lovable would only make sense if you wanted hosted sync across your own phone and a second device, which is more infrastructure than a personal fasting timer needs.

What will I need to maintain?

Very little. One or two files on a free static host, installed to your phone’s home screen.

Recurring work:

• Export a JSON or CSV backup every few months
• Reopen the app online occasionally so the cached PWA version stays current
• Re-grant notification permission if you switch phones

Should I build it, buy it, or reduce scope?

Build it if you use the timer and streak daily and want custom protocols or backfill rules Zero doesn’t offer.

Buy it if you actually use AI photo food logging, wearable sync, or the stage-view labels. Those are the parts of Plus with real engineering behind them, and rebuilding them yourself is a much bigger project than a timer.

Reduce scope if you just want to stop the upsell notifications. Turning off Zero’s promotional push notifications in your phone’s settings solves that without building anything.

Recommendation: build it if the timer, streak, and custom protocols are all you actually use, since that part is simple to replicate and free either way. Keep Zero Plus if photo food logging or wearable sync matter to you. If you’re also tracking sleep or water intake around your fasting window, see the sleep tracker build and the water tracker build.

What if I wanted to ship this to other people?

Shipping a fasting app means push notification infrastructure that works across devices, Apple Health and wearable OAuth integrations, and competing with an app that already gives the core loop away free. It’s a much bigger project than a personal timer, and the market already has a strong free option.

Starter build specification

• User: You, for your own use, tracking your own fasts.
• Problem: Wanting custom fasting protocols and streak rules without upsell nudges toward Zero Plus.
• Core workflow: Start a fast with a chosen protocol, watch the countdown, log water or coffee if needed, end the fast, see the streak update.
• Required features: Countdown timer, protocol switching, streak counter, backfill, edit and delete past fasts.
• Deliberate exclusions: No AI photo food logging, no wearable sync, no accounts, no social features.
• Data ownership: Stored in localStorage. Exported as JSON and CSV. Imported from JSON.
• Definition of done: Starting a fast that crosses midnight and a timezone change both display correctly, and the streak survives a normal gap between fasts without resetting incorrectly.

For scoping help, read can AI build an app for me.

Sources and verification

• How Much Does A Zero Plus Subscription Cost (Zero): $9.99/mo or $69.99/yr pricing.
• Exporting Your Fasting Data (Zero): the “Download My Data” export flow.
• Fasting Streaks (Zero): streak logic and notification behavior.
• Zero: Fasting & Food Tracker (App Store): free-tier and Plus feature breakdown.
• Is Zero Fasting App Worth It? 2026 Pricing & Features: third-party review of export format and reported bugs.

This app was not built and tested for this page. The verdict is based on Zero’s own help center articles, App Store listings, and a third-party review of the feature set. Zero’s price was checked on September 24, 2026.

Reviewed on September 24, 2026.