# Fix vue-i18n datetime format fallback locale missing error

## TL;DR

You defined datetime formats for the main locale but not the fallback, so add matching datetimeFormats for the fallback locale. When the active locale has no format for a key, vue-i18n falls back to the fallback locale, and if that has no format either it warns. Define the same format keys for every fallback locale you configure.

## The error

```text
[vue-i18n] Fall back to datetime format "short" of "en" but it is not defined
d(new Date(), 'short') with locale fr, fallbackLocale en
```

## Fix it

### Step 1: List the datetime formats you defined per locale

```bash
grep -n "datetimeFormats" -A 12 src/i18n.js | head -20
```

Expected: You see which locales have formats and which are missing.

### Step 2: Add the missing format keys to the fallback locale

```bash
node -e "console.log('add to datetimeFormats.en: short, long, whatever keys fr uses')"
```

Expected: Every format key used exists under the fallback locale too.

### Step 3: Reload and format a date in the secondary locale

```bash
node -e "console.log('call d(new Date(), \"short\") with locale fr, no warning appears')"
```

Expected: The fallback warning is gone and the date formats.

### Step 4: Audit all format keys across locales

```bash
node -e "console.log('script: collect format keys per locale, report any key missing from a fallback locale')"
```

Expected: No key is missing from any fallback locale.

## When to use this

- vue-i18n warns about falling back to a datetime format that is not defined
- Dates render in the wrong format for secondary locales

## When NOT to use this

- The format exists but the date is wrong, check the format pattern itself
- Number formats warn the same way, apply the same fix to numberFormats

## Tool and version compatibility

- vue-i18n v9 (Vue 3), datetimeFormats option
- Fallback chains with fallbackLocale set

## Variant phrasings

### same warning for number formats

Identical fix under numberFormats. Keep datetime and number format keys in sync across locales.

### fallback locale itself falls back further

Chains like fr to en to root need the format at the end of the chain. Define it on the final fallback.

## Why it happens

vue-i18n resolves formats per locale, then walks the fallback chain. The warning fires when the walk reaches a locale that also lacks the format key. Defining formats only for the primary locale leaves every fallback lookup empty.

## Edge cases

- Format keys are case sensitive, Short and short are different keys
- Lazy-loaded locales need their formats registered when the locale loads, not at init
- Custom format functions receive the fallback context, make them locale-agnostic

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_GEcXrjs4aQ6fvbE2YBknsg