# CalendarDuo vs M21

The clearest difference is ads: M21 at none against CalendarDuo's shown. M21 also leads on update cadence (every 6 weeks vs every 4 months). On price, in-app purchases, device support and iOS requirement there is nothing between them.

Both are free to download. Neither carries in-app purchases, so what you see is what you pay.

CalendarDuo ships an update every 4 months, M21 every 6 weeks. The most recent releases landed on June 24, 2026 and September 8, 2026 respectively.

## The numbers

| Metric | CalendarDuo | M21 |
| --- | --- | --- |
| Price | Free | Free |
| Positive reviews | 50.0% of reviews | — |
| In-app purchases | No | No |
| Ads | Yes | **No** |
| Update frequency | Every 4 months | **Every 6 weeks** |
| Monetization | Ad-supported | — |
| Size | 41 MB | 115 MB |
| Requires iOS | **15.1** | 15.5 |
| Devices | **iPhone, iPad, iPod** | iPhone, iPod |
| Age rating | 4+ | 4+ |
| Developer | [Chun Lam](https://appshunter.io/developer/1565498900) | [Telin](https://appshunter.io/developer/1225896354) |

## What reviewers say

### CalendarDuo

[App page](https://appshunter.io/ios/app/calendarduo/id1589593639)

Easily manage and view holidays from two different countries simultaneously. This utility app offers instant switching between calendars, a dual-view display, and helpful tools like a world clock and currency exchange.

**Highlights:** Switch between two world calendars instantly, Duo-view with holidays from two countries, Daylight Saving Time dates, Currency exchange rate, World clock, Add notes

**Features:** macOS Compatibility

**In-app purchases:** none


### M21

[App page](https://appshunter.io/ios/app/m21/id1671477072)

**In-app purchases:** none


## Frequently asked questions

### Is CalendarDuo free?

CalendarDuo is free to download, with no in-app purchases.

### Is M21 free?

M21 is free to download, with no in-app purchases.

### Do CalendarDuo or M21 have ads?

CalendarDuo shows ads; M21 does not.

### Which is updated more often, CalendarDuo or M21?

CalendarDuo ships an update every 4 months, and M21 every 6 weeks. Most recently, CalendarDuo was updated on June 24, 2026 and M21 on September 8, 2026.

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*Compared by [AppsHunter](https://appshunter.io/) from daily US App Store data. Review summaries are generated from verbatim App Store reviews.*