diff --git a/src/utils/request.ts b/src/utils/request.ts
index f5293a8..56fcf09 100644
--- a/src/utils/request.ts
+++ b/src/utils/request.ts
@@ -30,13 +30,7 @@ export function parseUrl(request: Request): URL | undefined {
 }
 
 export function parseConditionalHeaders(headers: Headers): ConditionalHeaders {
-  const ifModifiedSince = headers.has('if-modified-since')
-    ? new Date(headers.get('if-modified-since')!)
-    : undefined;
-
-  if (ifModifiedSince instanceof Date) {
-    ifModifiedSince.setSeconds(ifModifiedSince.getSeconds() + 1);
-  }
+  const ifModifiedSince = getDateFromHeader(headers.get('if-modified-since'));
 
   const ifMatch = headers.has('if-match')
     ? headers.get('if-match')!.replaceAll('"', '')
@@ -46,9 +40,9 @@ export function parseConditionalHeaders(headers: Headers): ConditionalHeaders {
     ? headers.get('if-none-match')!.replaceAll('"', '')
     : undefined;
 
-  const ifUnmodifiedSince = headers.has('if-unmodified-since')
-    ? new Date(headers.get('if-unmodified-since')!)
-    : undefined;
+  const ifUnmodifiedSince = getDateFromHeader(
+    headers.get('if-unmodified-since')
+  );
 
   const range = headers.has('range')
     ? parseRangeHeader(headers.get('range')!)
@@ -63,6 +57,15 @@ export function parseConditionalHeaders(headers: Headers): ConditionalHeaders {
   };
 }
 
+function getDateFromHeader(dateString: string | null): Date | undefined {
+  if (dateString === null) {
+    return undefined;
+  }
+
+  const date = new Date(dateString);
+  return !isNaN(date.getTime()) ? date : undefined;
+}
+
 /**
  * @see https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Range
  * @returns undefined if header is invalid
diff --git a/tests/e2e/file.test.ts b/tests/e2e/file.test.ts
index 64ea892..9a5fb40 100644
--- a/tests/e2e/file.test.ts
+++ b/tests/e2e/file.test.ts
@@ -78,11 +78,14 @@ describe('File Tests', () => {
     assert.strictEqual(res.status, 200);
     assert.strictEqual(res.headers.has('last-modified'), true);
 
+    const date = new Date(res.headers.get('last-modified')!);
+    date.setSeconds(date.getSeconds() + 1);
+
     // Make sure it returns a 304 when If-Modified-Since
     //  >= file last modified
     res = await mf.dispatchFetch(`${url}dist/index.json`, {
       headers: {
-        'if-modified-since': res.headers.get('last-modified')!,
+        'if-modified-since': date.toUTCString(),
       },
     });
     assert.strictEqual(res.status, 304);
diff --git a/tests/unit/utils/request.test.ts b/tests/unit/utils/request.test.ts
index bab0398..dba7372 100644
--- a/tests/unit/utils/request.test.ts
+++ b/tests/unit/utils/request.test.ts
@@ -1,6 +1,9 @@
 import assert from 'node:assert';
 import { describe, it } from 'node:test';
-import { parseRangeHeader } from '../../../src/utils/request';
+import {
+  parseConditionalHeaders,
+  parseRangeHeader,
+} from '../../../src/utils/request';
 
 describe('parseRangeHeader', () => {
   it('`bytes=0-10`', () => {
@@ -49,3 +52,20 @@ describe('parseRangeHeader', () => {
     assert.strictEqual(result, undefined);
   });
 });
+
+describe('parseConditionalHeaders', () => {
+  it('invalid dates', () => {
+    const headers = new Headers({
+      'if-modified-since': 'asd',
+      'if-unmodified-since': 'asd',
+    });
+
+    assert.deepStrictEqual(parseConditionalHeaders(headers), {
+      ifMatch: undefined,
+      ifNoneMatch: undefined,
+      ifModifiedSince: undefined,
+      ifUnmodifiedSince: undefined,
+      range: undefined,
+    });
+  });
+});