diff --git a/Libraries/LibJS/Runtime/DateParser.h b/Libraries/LibJS/Runtime/DateParser.h
index cc34f58df6..7a474f5502 100644
--- a/Libraries/LibJS/Runtime/DateParser.h
+++ b/Libraries/LibJS/Runtime/DateParser.h
@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <AK/CharacterTypes.h>
 #include <AK/Error.h>
 #include <AK/GenericLexer.h>
 #include <AK/Optional.h>
@@ -15,8 +16,6 @@
 #include <AK/Vector.h>
 #include <LibJS/Runtime/Date.h>
 
-#include <cctype> // Not included by default on macOS.
-
 // Parse simplified ISO8601 and non-standard date formats to milliseconds
 // from epoch (double). Synopsis:
 // (1) Try to parse the string as simplified ISO8601 (case sensitive).
@@ -50,6 +49,8 @@ class DateParser : public GenericLexer {
 public:
     ALWAYS_INLINE static double parse(StringView string)
     {
+        if (!string.is_ascii())
+            return NAN;
         return DateParser(string).parse().value_or(NAN);
     }
 
@@ -107,13 +108,13 @@ private:
         ParsedNumber<T> result;
 
         for (; result.digits < MaxLength; ++result.digits) {
-            if (is_eof() || !next_is(isdigit))
+            if (is_eof() || !next_is(is_ascii_digit))
                 return result;
             result.number *= 10;
             result.number += consume() - '0';
         }
 
-        ignore_while(isdigit);
+        ignore_while(is_ascii_digit);
         return result;
     }
 
@@ -331,7 +332,7 @@ private:
         case '(':
             ignore_until(')'); // Consume time zone name (Anything in brackets).
             ignore();
-            ignore_while(isspace);
+            ignore_while(is_ascii_space);
             return true;
         }
 
@@ -342,7 +343,7 @@ private:
     {
         VERIFY(m_hours.has_value());
 
-        consume_while(isspace);
+        consume_while(is_ascii_space);
 
         if (consume_specific("AM"sv)) {
             if (!separator())
@@ -588,7 +589,7 @@ private:
 
         m_timezone_utc = true;
 
-        bool space = consume_while(isspace).length() > 0;
+        bool space = consume_while(is_ascii_space).length() > 0;
         switch (peek()) {
         case '+':
         case '-':
@@ -622,7 +623,7 @@ private:
             if (str[i] != peek(i))
                 return false;
 
-        ignore_while(isalpha); // ... which can followed by anything. Just like Firefox and Chrome.
+        ignore_while(is_ascii_alpha); // ... which can followed by anything. Just like Firefox and Chrome.
         m_month = month;
 
         return separator(); // Must end with a separator.
@@ -630,7 +631,7 @@ private:
 
     ALWAYS_INLINE bool word() // Alphanumeric strings that are not date "keywords".
     {
-        consume_while(isalpha);
+        consume_while(is_ascii_alpha);
         // Just like Firefox and Chrome:
         // - Ignore junk (bare words) at the beginning (before time or a date fragment has been read).
         // - Fail if a word is read later in the date string (exception: final time zone name, in brackets).
diff --git a/Tests/LibJS/Runtime/builtins/Date/Date.parse.nonStandard.js b/Tests/LibJS/Runtime/builtins/Date/Date.parse.nonStandard.js
index c12a381431..1b3ecf7cf1 100644
--- a/Tests/LibJS/Runtime/builtins/Date/Date.parse.nonStandard.js
+++ b/Tests/LibJS/Runtime/builtins/Date/Date.parse.nonStandard.js
@@ -698,3 +698,13 @@ test("multiple month names", () => {
 
     setTimeZone(originalTimeZone);
 });
+
+test("non-ASCII input", () => {
+    expect(Date.parse("2024\uff01")).toBeNaN();
+    expect(Date.parse("1234567\uff01")).toBeNaN();
+    expect(Date.parse("Jan\uff01 01 2024")).toBeNaN();
+    expect(Date.parse("hello\uff01 Jan 01 2024")).toBeNaN();
+    expect(Date.parse("Jan 01 2024 12:34\u00a0PM")).toBeNaN();
+    expect(Date.parse("Jan 01 2024 GMT\u00a0")).toBeNaN();
+    expect(Date.parse("Jan 01 2024 (UTC)\u00a0")).toBeNaN();
+});