Fix: Use locale to disambiguate date format during CSV import (#107)

## Summary
- When CSV dates are ambiguous (e.g. `05/03/2024` parses as both
DD-MM-YYYY and MM-DD-YYYY), the auto-detection heuristic now uses the
user's locale preference as a tie-breaker instead of relying on array
order
- Adds `getLocaleDateFormat()` helper that maps locale strings to their
expected date format (US → MM-DD-YYYY, most others → DD-MM-YYYY, CJK →
YYYY-MM-DD)
- Passes locale from Inertia shared props to both transaction and
balance import flows

Closes #100

## Test plan
- [x] Added 15 Vitest tests for `getLocaleDateFormat` and
`autoDetectDateFormat` with locale tie-breaking
- [ ] Manual test: Import CSV with ambiguous dates (day ≤ 12) using a
non-US locale, verify DD-MM-YYYY is preferred
- [ ] Manual test: Import CSV with unambiguous dates (day > 12), verify
locale does NOT override correct detection

resources/js/components/accounts/import-balances-drawer.tsx

@@ -18,6 +18,7 @@ import {
     parseDate,
     parseFile,
 } from '@/lib/file-parser';
+import { type SharedData } from '@/types';
 import { type Account } from '@/types/account';
 import {
     BalanceImportStep,
@@ -28,6 +29,7 @@ import {
 import { DateFormat } from '@/types/import';
 import type { UUID } from '@/types/uuid';
 import { __ } from '@/utils/i18n';
+import { usePage } from '@inertiajs/react';
 import { Check } from 'lucide-react';
 import { useEffect, useState } from 'react';
 import { toast } from 'sonner';
@@ -60,6 +62,7 @@ export function ImportBalancesDrawer({
     accountId,
     onSuccess,
 }: ImportBalancesDrawerProps) {
+    const { locale } = usePage<SharedData>().props;
     const [isImporting, setIsImporting] = useState(false);
     const [importProgress, setImportProgress] = useState(0);
     const [importTotal, setImportTotal] = useState(0);
@@ -240,6 +243,7 @@ export function ImportBalancesDrawer({
                 const detected = autoDetectDateFormat(
                     data,
                     autoMapping.balance_date,
+                    locale,
                 );
                 if (detected) {
                     detectedFormat = detected;

resources/js/components/transactions/import-transactions-drawer.tsx

@@ -22,6 +22,7 @@ import {
 import { getStoredKey } from '@/lib/key-storage';
 import { evaluateRulesForNewTransaction } from '@/lib/rule-engine';
 import { transactionSyncService } from '@/services/transaction-sync';
+import { type SharedData } from '@/types';
 import { type Account, type Bank } from '@/types/account';
 import { type AutomationRule } from '@/types/automation-rule';
 import { type Category } from '@/types/category';
@@ -31,7 +32,6 @@ import {
     type ColumnMapping,
     type ImportState,
 } from '@/types/import';
-import { type SharedData } from '@/types';
 import { __ } from '@/utils/i18n';
 import { router, usePage } from '@inertiajs/react';
 import { useEffect, useState } from 'react';
@@ -71,7 +71,7 @@ export function ImportTransactionsDrawer({
     onImportComplete,
 }: ImportTransactionsDrawerProps) {
     const { isKeySet } = useEncryptionKey();
-    const { features } = usePage<SharedData>().props;
+    const { features, locale } = usePage<SharedData>().props;
     const isPlaintext = features['plaintext-transactions'];
     const [isImporting, setIsImporting] = useState(false);
     const [importProgress, setImportProgress] = useState(0);
@@ -187,6 +187,7 @@ export function ImportTransactionsDrawer({
                 const detected = autoDetectDateFormat(
                     data,
                     autoMapping.transaction_date,
+                    locale,
                 );
                 if (detected) {
                     detectedFormat = detected;

resources/js/lib/file-parser.test.ts

@@ -0,0 +1,118 @@
+import { DateFormat } from '@/types/import';
+import { describe, expect, it } from 'vitest';
+import { autoDetectDateFormat, getLocaleDateFormat } from './file-parser';
+
+describe('getLocaleDateFormat', () => {
+    it('returns null for undefined locale', () => {
+        expect(getLocaleDateFormat(undefined)).toBeNull();
+    });
+
+    it('returns MM-DD-YYYY for en-US', () => {
+        expect(getLocaleDateFormat('en-US')).toBe(DateFormat.MonthDayYear);
+    });
+
+    it('returns DD-MM-YYYY for en-GB', () => {
+        expect(getLocaleDateFormat('en-GB')).toBe(DateFormat.DayMonthYear);
+    });
+
+    it('returns DD-MM-YYYY for es', () => {
+        expect(getLocaleDateFormat('es')).toBe(DateFormat.DayMonthYear);
+    });
+
+    it('returns DD-MM-YYYY for de', () => {
+        expect(getLocaleDateFormat('de')).toBe(DateFormat.DayMonthYear);
+    });
+
+    it('returns DD-MM-YYYY for fr', () => {
+        expect(getLocaleDateFormat('fr')).toBe(DateFormat.DayMonthYear);
+    });
+
+    it('handles underscored locales like en_US', () => {
+        expect(getLocaleDateFormat('en_US')).toBe(DateFormat.MonthDayYear);
+    });
+});
+
+describe('autoDetectDateFormat', () => {
+    it('returns null for empty data', () => {
+        expect(autoDetectDateFormat([], 'date')).toBeNull();
+    });
+
+    it('detects YYYY-MM-DD unambiguously', () => {
+        const data = [
+            { date: '2024-01-15' },
+            { date: '2024-02-20' },
+            { date: '2024-03-25' },
+        ];
+        expect(autoDetectDateFormat(data, 'date')).toBe(
+            DateFormat.YearMonthDay,
+        );
+    });
+
+    it('detects DD-MM-YYYY when day > 12 disambiguates', () => {
+        const data = [
+            { date: '15/01/2024' },
+            { date: '20/02/2024' },
+            { date: '25/03/2024' },
+        ];
+        expect(autoDetectDateFormat(data, 'date')).toBe(
+            DateFormat.DayMonthYear,
+        );
+    });
+
+    it('detects MM-DD-YYYY when day > 12 disambiguates', () => {
+        const data = [
+            { date: '01/15/2024' },
+            { date: '02/20/2024' },
+            { date: '03/25/2024' },
+        ];
+        expect(autoDetectDateFormat(data, 'date')).toBe(
+            DateFormat.MonthDayYear,
+        );
+    });
+
+    it('uses locale to break tie for ambiguous dates (en-GB prefers DD-MM-YYYY)', () => {
+        // All dates have day <= 12, so DD-MM-YYYY and MM-DD-YYYY both parse
+        const data = [
+            { date: '05/03/2024' },
+            { date: '06/04/2024' },
+            { date: '07/05/2024' },
+        ];
+        expect(autoDetectDateFormat(data, 'date', 'en-GB')).toBe(
+            DateFormat.DayMonthYear,
+        );
+    });
+
+    it('uses locale to break tie for ambiguous dates (en-US prefers MM-DD-YYYY)', () => {
+        const data = [
+            { date: '05/03/2024' },
+            { date: '06/04/2024' },
+            { date: '07/05/2024' },
+        ];
+        expect(autoDetectDateFormat(data, 'date', 'en-US')).toBe(
+            DateFormat.MonthDayYear,
+        );
+    });
+
+    it('uses locale to break tie for ambiguous dates (es prefers DD-MM-YYYY)', () => {
+        const data = [
+            { date: '05/03/2024' },
+            { date: '06/04/2024' },
+            { date: '07/05/2024' },
+        ];
+        expect(autoDetectDateFormat(data, 'date', 'es')).toBe(
+            DateFormat.DayMonthYear,
+        );
+    });
+
+    it('prefers unambiguous detection over locale', () => {
+        // Day > 12, so only DD-MM-YYYY parses correctly, even with en-US locale
+        const data = [
+            { date: '15/01/2024' },
+            { date: '20/02/2024' },
+            { date: '25/03/2024' },
+        ];
+        expect(autoDetectDateFormat(data, 'date', 'en-US')).toBe(
+            DateFormat.DayMonthYear,
+        );
+    });
+});

resources/js/lib/file-parser.ts

@@ -178,9 +178,43 @@ export async function parseFile(file: File): Promise<{
     });
 }
 
+/**
+ * Returns the preferred date format for a given locale.
+ * Most locales use DD-MM-YYYY, while US/Philippines/etc use MM-DD-YYYY.
+ */
+export function getLocaleDateFormat(locale?: string): DateFormat | null {
+    if (!locale) {
+        return null;
+    }
+
+    const mdyLocales = ['en-US', 'en-PH', 'fil', 'ja', 'zh', 'ko', 'hu'];
+    const ymdLocales = [
+        'sv',
+        'lt',
+        'zh-CN',
+        'zh-TW',
+        'ja-JP',
+        'ko-KR',
+        'hu-HU',
+    ];
+
+    const normalized = locale.replace('_', '-');
+
+    if (ymdLocales.some((l) => normalized.startsWith(l))) {
+        return DateFormat.YearMonthDay;
+    }
+
+    if (mdyLocales.some((l) => normalized.startsWith(l))) {
+        return DateFormat.MonthDayYear;
+    }
+
+    return DateFormat.DayMonthYear;
+}
+
 export function autoDetectDateFormat(
     data: ParsedRow[],
     dateColumnName: string,
+    locale?: string,
 ): DateFormat | null {
     if (!data || data.length === 0 || !dateColumnName) {
         return null;
@@ -216,10 +250,22 @@ export function autoDetectDateFormat(
         return null;
     }
 
-    const bestFormat = formats.find((format) => scores[format] === maxScore);
-
     if (maxScore >= sampleSize * 0.8) {
-        return bestFormat || null;
+        const tiedFormats = formats.filter(
+            (format) => scores[format] === maxScore,
+        );
+
+        if (tiedFormats.length === 1) {
+            return tiedFormats[0];
+        }
+
+        // Use the user's locale to break ties between ambiguous formats
+        const localePreferred = getLocaleDateFormat(locale);
+        if (localePreferred && tiedFormats.includes(localePreferred)) {
+            return localePreferred;
+        }
+
+        return tiedFormats[0];
     }
 
     return null;