fix: restore native Date fallback when strictParsing is false

When strictParsing is false (the default), restore the native Date
parsing fallback that was present in v7.x but accidentally removed
in v8.0.0. This allows flexible input formats like "12/05/2025" or
"2025-12-16 3:31:01 PM" to be parsed even when the dateFormat prop
specifies a different format.

The fallback only triggers when:
1. strictParsing is false (default)
2. The input is at least 8 characters (to avoid parsing partial inputs)
3. date-fns parsing with the specified format(s) failed

This fixes the regression where v8.x required exact format matches
even with strictParsing disabled.

Fixes #6164

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: martijnrusschen

src/date_utils.ts

@@ -318,6 +318,20 @@ export function parseDate(
       return parsedDate;
     }
   }
+
+  // When strictParsing is false, try native Date parsing as a fallback
+  // This allows flexible input formats like "12/05/2025" or "2025-12-16"
+  // even when the dateFormat prop specifies a different format.
+  // Only attempt this for inputs that look like complete dates (minimum
+  // length of 8 characters, e.g., "1/1/2000") to avoid parsing partial
+  // inputs like "03/" or "2000" which should be handled by parseDateForNavigation.
+  if (!strictParsing && value && value.length >= 8) {
+    const nativeDate = new Date(value);
+    if (isValidDate(nativeDate)) {
+      return nativeDate;
+    }
+  }
+
   return null;
 }
 

src/test/date_utils_test.test.ts

@@ -1094,7 +1094,10 @@ describe("date_utils", () => {
       expect(parseDate(valueEn, dateFormat, undefined, false)).toEqual(
         expected,
       );
-      expect(parseDate(valueEn, dateFormat, locale, false)).toBeNull();
+      // With native Date fallback (restored in v8.x), English month names
+      // are parsed even when a different locale is specified, because
+      // the native Date constructor is locale-agnostic
+      expect(parseDate(valueEn, dateFormat, locale, false)).toEqual(expected);
     });
 
     it("should not parse date based on locale without a given locale", () => {
@@ -1117,6 +1120,63 @@ describe("date_utils", () => {
 
       expect(actual).toEqual(expected);
     });
+
+    describe("native Date fallback when strictParsing is false (#6164)", () => {
+      it("should parse date in different format using native Date fallback", () => {
+        // User types MM/dd/yyyy but dateFormat is yyyy-MM-dd
+        const value = "12/05/2025";
+        const dateFormat = "yyyy-MM-dd";
+
+        const result = parseDate(value, dateFormat, undefined, false);
+        expect(result).not.toBeNull();
+        expect(result?.getFullYear()).toBe(2025);
+        expect(result?.getMonth()).toBe(11); // December (0-indexed)
+        expect(result?.getDate()).toBe(5);
+      });
+
+      it("should parse ISO date string using native Date fallback", () => {
+        const value = "2025-12-16";
+        const dateFormat = "MM/dd/yyyy";
+
+        const result = parseDate(value, dateFormat, undefined, false);
+        expect(result).not.toBeNull();
+        expect(result?.getFullYear()).toBe(2025);
+        expect(result?.getMonth()).toBe(11); // December
+        expect(result?.getDate()).toBe(16);
+      });
+
+      it("should parse date with time using native Date fallback", () => {
+        const value = "2025-12-16 3:31:01 PM";
+        const dateFormat = "yyyy-MM-dd hh:mm aa";
+
+        const result = parseDate(value, dateFormat, undefined, false);
+        expect(result).not.toBeNull();
+        expect(result?.getFullYear()).toBe(2025);
+        expect(result?.getMonth()).toBe(11); // December
+        expect(result?.getDate()).toBe(16);
+        expect(result?.getHours()).toBe(15); // 3 PM = 15:00
+        expect(result?.getMinutes()).toBe(31);
+      });
+
+      it("should NOT use native Date fallback when strictParsing is true", () => {
+        const value = "12/05/2025";
+        const dateFormat = "yyyy-MM-dd";
+
+        const result = parseDate(value, dateFormat, undefined, true);
+        expect(result).toBeNull();
+      });
+
+      it("should prefer exact format match over native Date fallback", () => {
+        const value = "2025-12-05";
+        const dateFormat = "yyyy-MM-dd";
+
+        const result = parseDate(value, dateFormat, undefined, false);
+        expect(result).not.toBeNull();
+        expect(result?.getFullYear()).toBe(2025);
+        expect(result?.getMonth()).toBe(11); // December
+        expect(result?.getDate()).toBe(5);
+      });
+    });
   });
 
   describe("isMonthInRange", () => {