diff --git a/Tests/LibWeb/CMakeLists.txt b/Tests/LibWeb/CMakeLists.txt
index b62aa719c1..440c01dba4 100644
--- a/Tests/LibWeb/CMakeLists.txt
+++ b/Tests/LibWeb/CMakeLists.txt
@@ -2,6 +2,7 @@ set(TEST_SOURCES
     TestCSSIDSpeed.cpp
     TestCSSPixels.cpp
     TestHTMLTokenizer.cpp
+    TestMicrosyntax.cpp
     TestMimeSniff.cpp
     TestNumbers.cpp
 )
diff --git a/Tests/LibWeb/TestMicrosyntax.cpp b/Tests/LibWeb/TestMicrosyntax.cpp
new file mode 100644
index 0000000000..0629598d1a
--- /dev/null
+++ b/Tests/LibWeb/TestMicrosyntax.cpp
@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2023, stelar7 <dudedbz@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+#include <LibWeb/HTML/Dates.h>
+
+TEST_CASE(week_number_of_the_last_day)
+{
+    EXPECT_EQ(Web::HTML::week_number_of_the_last_day(1998), (u32)53);
+    EXPECT_EQ(Web::HTML::week_number_of_the_last_day(1999), (u32)52);
+    EXPECT_EQ(Web::HTML::week_number_of_the_last_day(2000), (u32)52);
+    EXPECT_EQ(Web::HTML::week_number_of_the_last_day(2001), (u32)52);
+    EXPECT_EQ(Web::HTML::week_number_of_the_last_day(2002), (u32)52);
+    EXPECT_EQ(Web::HTML::week_number_of_the_last_day(2003), (u32)52);
+    EXPECT_EQ(Web::HTML::week_number_of_the_last_day(2004), (u32)53);
+    EXPECT_EQ(Web::HTML::week_number_of_the_last_day(2005), (u32)52);
+    EXPECT_EQ(Web::HTML::week_number_of_the_last_day(2006), (u32)52);
+    EXPECT_EQ(Web::HTML::week_number_of_the_last_day(2007), (u32)52);
+    EXPECT_EQ(Web::HTML::week_number_of_the_last_day(2008), (u32)52);
+    EXPECT_EQ(Web::HTML::week_number_of_the_last_day(2009), (u32)53);
+}