Can AI build a family calendar app like Cozi?

Build a personal family calendar web app to replace Cozi for my own use. I'm the primary person managing it, and other family members may view it on their own devices.

Tech rules:
- Plain HTML, CSS, and vanilla JavaScript in one file. No framework or build step.
- Store all data in localStorage under one key, with a version number, as the default. If real-time sync across devices matters to you, note that as a separate stretch goal requiring a small hosted database, not local-only storage.

Data model:
- FamilyMember: id, name, color.
- Event: id, memberIds [], title, startAt, endAt, allDay, recurrenceRule (none, weekly, monthly, custom), seriesId (for recurring events), notes.
- ListItem: id, listId, text, checked, addedBy.
- List: id, name, type (shopping, todo).

Features:
- Calendar with day, week, and month views. Events are color-coded by family member.
- Add a recurring event (like a weekly sports practice). Editing offers a choice: this event only, or the whole series.
- No cap on how far in advance I can add events.
- Shared shopping and to-do lists. Any item can be checked off and cleared.
- A simple view filtered to one family member's events only.

Edge cases the build must handle correctly:
1. Multiple family members can have color-coded calendar events on the same day without visually overlapping or hiding each other.
2. Shared shopping and to-do lists update correctly when items are added, checked, or removed, even if edited from two browser tabs.
3. Recurring events like school schedules and sports practices can be edited as a single instance without changing the whole series, and vice versa.
4. The calendar has no artificial cap on how far ahead I can add an event, unlike Cozi's free 30-day window.
5. Switching between day, week, and month views keeps the same selected date in context.
6. Assigning a task or chore to a specific family member is visible on their filtered view.
7. Deleting a family member reassigns or clearly marks their past events instead of deleting history silently.

Backup:
- One-click "Export JSON" of all events, lists, and family members.
- "Import JSON" restores after confirmation.

Layout:
- Mobile-first, readable at 375px wide, with the color-coded legend visible without scrolling.

Exclusions:
- No publishing, App Store submission, real-time push notifications, payments, recipes, or meal-planning features.

Write a short README explaining where data is stored, how to export and restore it, and what real-time multi-device sync would require if I want to add it later.

Keep it under 700 lines. This is a personal tool, not a product.

What is the short answer?

Yes, but. AI can build a family calendar app that replaces Cozi Gold at $39 per year with no cap on how far ahead you can plan. That specific limitation is real: Cozi’s own FAQ confirms free users can only add new calendar events within a rolling 30-day window from Agenda view, a cap that users say broke pre-entered school-year and travel bookings when it was added.

The “but” is sync. Cozi’s free tier already gives every family member a live, shared calendar and lists across their own devices. A local web app can match the calendar and list features, but keeping them in sync in real time across everyone’s phones is the harder engineering problem, not the easier one.

Option	Best for	Upfront cost	Ongoing cost	Main limitation
Build a rough prototype	One person who wants a calendar and lists with no forward-planning cap	$0 and 3–4 hours	$0	Only updates on the device you’re using; no live sync
Build a daily-use personal version	A primary planner who wants recurring events, chores, and lists done right	$0–$20 and 1–2 days	$0–$5	Still needs manual steps or a small backend for other family members to see live updates
Keep paying for Cozi Gold	Families who want automatic real-time sync across everyone’s devices	$0	$39/year (Max: $79/year)	Free tier’s 30-day planning cap is the reason most people search this page

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

What should the first version include?

The first version needs to hold a full family’s schedule without an artificial cap on how far ahead you can plan.

• Color-coded calendar events per family member
• Shared shopping and to-do lists
• Recurring events for school and sports schedules
• Day, week, and month views
• No 30-day forward-planning cap
• Deliberate exclusion: no real-time push notifications in the first version. Start with a calendar you check, not one that pings everyone.

What can AI build reliably?

ChatGPT and Claude build calendar and list logic reliably. The data model, color-coding, and view switching are well-trodden patterns.

Concrete things AI gets right:

• Day, week, and month views that stay on the same selected date when you switch
• Color-coded events per family member that don’t visually collide
• Recurring event logic that lets you edit one instance or the whole series
• Shared lists where items can be added, checked, and cleared

Where will AI need human help?

Sync across devices is the real limitation, and it’s the row’s central point: real-time multi-user sync across devices and family members is genuinely harder than a single-user app. Cozi solves this with its own servers. A local web app only reliably updates on the device it’s open on, unless you add a small hosted backend.

Push notifications for shared updates need a backend service, not just client-side code. If you skip a backend, the tradeoff is that family members won’t get pinged when something changes; they’ll need to open the app to see updates.

Recurring events are a classic trap. AI’s first pass often changes an entire series when you meant to move one Tuesday’s practice. Test this specifically: add a weekly event, edit a single occurrence, and confirm the rest of the series is untouched.

The decision only you can make is whether one person managing the calendar, with others viewing a shared link or export, is good enough. If every family member needs to add and edit in real time from their own phone, that’s a reason to keep Cozi, not a bug to fix.

Can I get my data out of Cozi?

Barely, and mostly not. No native CSV or file export exists for any Cozi data. The only official way out is a live, read-only iCal subscription URL, found under Settings > Shared Cozi Calendars, that Google Calendar, Outlook, or Apple Calendar can subscribe to. It only carries calendar events, and updates on the receiving calendar’s own refresh cycle, which for Google can take up to 48 hours.

The trap: that read-only feed covers calendar events only. Shopping lists, to-do lists, recipes, and journal entries have no export path at all. It’s a live subscription link, not a downloadable snapshot, so there’s no one-time backup file to grab before canceling.

The practical path: subscribe to the iCal feed from your calendar app while your Cozi account is still active, so your event history lands somewhere you control. For lists and recipes, manually copy what matters before you cancel, since nothing will be recoverable after.

How long will it take and what will it cost?

Rough prototype: 3–4 hours and $0. A working calendar with color-coded events, recurring logic, and shared lists, all on one device.

Dependable daily-use version: 1–2 days, including recurring-event editing, chore assignment, and either manual export/import between devices or a small hosted sync backend.

Cozi Gold costs $39 per year, about $3.25 per month. A $20 paid AI month is roughly 6 months of Gold. Build it if the 30-day planning cap genuinely bothers you or you want features Cozi doesn’t have, not primarily to save money.

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 the calendar and list logic in one or two conversations.

Cursor is useful if you want to iterate on the recurring-event edge cases carefully, since that logic benefits from stepping through code.

Lovable is the realistic route if real-time sync across every family member’s device is the actual goal. That means a hosted database, which is a bigger commitment than a local file, in exchange for the one feature Cozi does well out of the box.

What will I need to maintain?

Modest, depending on whether you add sync.

Recurring work:

• Export a JSON backup periodically, since there’s no server copy of your data
• Re-check recurring events after any manual edit to a series
• If you added a hosted backend for sync, keep an eye on its free-tier limits

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

Build it if you’re the primary planner, don’t need every family member editing live, and want a calendar with no forward-planning cap.

Buy it if your family genuinely needs real-time updates on everyone’s phone. Cozi’s $39/year Gold plan, or $79/year Max, buys sync infrastructure that’s expensive to replicate for personal use.

Reduce scope if you only need a shared calendar, not lists or recipes. A shared Google Calendar or Apple Calendar, which every family member already has, does color-coded multi-person scheduling for free.

Recommendation: keep Cozi if more than one person needs to add events or check off list items independently through the week; that live sync is the product. Build your own if you’re the one holding the calendar together and just want the 30-day cap gone. For recipes and meal planning that Cozi bundles in, the Paprika build handles that separately, and family budgeting pairs well with the YNAB-style budget build.

What if I wanted to ship this to other people?

Shipping a family calendar means real-time sync infrastructure, accounts for every family member, conflict resolution when two people edit the same event, and competing with Cozi’s established free tier. It’s a much bigger project than a calendar for your own household.

Starter build specification

• User: You, as the primary planner for your family, for your own use.
• Problem: Cozi’s free tier caps how far ahead you can add calendar events, and Gold costs $39/year to remove that cap.
• Core workflow: Add events and recurring schedules, check items off shared lists, view by day, week, or month.
• Required features: Color-coded events per family member, recurring event editing, shared shopping and to-do lists, no forward-planning cap.
• Deliberate exclusions: No real-time push notifications, no recipes, no meal planning, no accounts.
• Data ownership: Stored in localStorage. Exported and imported as JSON.
• Definition of done: Adding an event 6 months out works without error, and editing one instance of a recurring event leaves the rest of the series unchanged.

For scoping help, read can AI build an app for me. For daily routines beyond the calendar, the habit tracker build and a Day One-style journal cover adjacent ground.

Sources and verification

• Cozi plan comparison: free, Gold, and Max pricing.
• Cozi Gold and Cozi Gold features: what Gold adds over free.
• Cozi FAQ: confirms the free tier’s 30-day forward-planning cap on new events.
• Using Cozi with other calendars: the read-only iCal export method and its limits.

This app was not built and tested for this page. The verdict is based on Cozi’s own pricing pages and FAQ. Cozi’s price was checked on September 24, 2026.

Reviewed on September 24, 2026.