diff --git a/X10D.Tests/src/Time/DateTimeOffsetTests.cs b/X10D.Tests/src/Time/DateTimeOffsetTests.cs
index d79ac4d..eb2d937 100644
--- a/X10D.Tests/src/Time/DateTimeOffsetTests.cs
+++ b/X10D.Tests/src/Time/DateTimeOffsetTests.cs
@@ -37,6 +37,33 @@ public class DateTimeOffsetTests
         Assert.AreEqual(new DateTime(today.Year, today.Month, 1), today.FirstDayOfMonth());
     }
 
+    [TestMethod]
+    public void GetIso8601WeekOfYear_ShouldReturn1_Given4January1970()
+    {
+        DateTimeOffset date = new DateTime(1970, 1, 4);
+        int iso8601WeekOfYear = date.GetIso8601WeekOfYear();
+
+        Assert.AreEqual(1, iso8601WeekOfYear);
+    }
+
+    [TestMethod]
+    public void GetIso8601WeekOfYear_ShouldReturn1_Given31December1969()
+    {
+        DateTimeOffset date = new DateTime(1969, 12, 31);
+        int iso8601WeekOfYear = date.GetIso8601WeekOfYear();
+
+        Assert.AreEqual(1, iso8601WeekOfYear);
+    }
+
+    [TestMethod]
+    public void GetIso8601WeekOfYear_ShouldReturn53_Given31December1970()
+    {
+        DateTimeOffset date = new DateTime(1970, 12, 31);
+        int iso8601WeekOfYear = date.GetIso8601WeekOfYear();
+
+        Assert.AreEqual(53, iso8601WeekOfYear);
+    }
+
     [TestMethod]
     public void IsLeapYear_ShouldBeFalse_Given1999()
     {
diff --git a/X10D.Tests/src/Time/DateTimeTests.cs b/X10D.Tests/src/Time/DateTimeTests.cs
index b6c3174..d08b78e 100644
--- a/X10D.Tests/src/Time/DateTimeTests.cs
+++ b/X10D.Tests/src/Time/DateTimeTests.cs
@@ -37,6 +37,33 @@ public class DateTimeTests
         Assert.AreEqual(new DateTime(today.Year, today.Month, 1), today.FirstDayOfMonth());
     }
 
+    [TestMethod]
+    public void GetIso8601WeekOfYear_ShouldReturn1_Given4January1970()
+    {
+        var date = new DateTime(1970, 1, 4);
+        int iso8601WeekOfYear = date.GetIso8601WeekOfYear();
+
+        Assert.AreEqual(1, iso8601WeekOfYear);
+    }
+
+    [TestMethod]
+    public void GetIso8601WeekOfYear_ShouldReturn1_Given31December1969()
+    {
+        var date = new DateTime(1969, 12, 31);
+        int iso8601WeekOfYear = date.GetIso8601WeekOfYear();
+
+        Assert.AreEqual(1, iso8601WeekOfYear);
+    }
+
+    [TestMethod]
+    public void GetIso8601WeekOfYear_ShouldReturn53_Given31December1970()
+    {
+        var date = new DateTime(1970, 12, 31);
+        int iso8601WeekOfYear = date.GetIso8601WeekOfYear();
+
+        Assert.AreEqual(53, iso8601WeekOfYear);
+    }
+
     [TestMethod]
     public void IsLeapYear_ShouldBeFalse_Given1999()
     {