diff --git a/poi/src/test/java/org/apache/poi/ss/formula/functions/TestWeekNumFunc.java b/poi/src/test/java/org/apache/poi/ss/formula/functions/TestWeekNumFunc.java
index 9ccd18dc9d..4e801a657c 100644
--- a/poi/src/test/java/org/apache/poi/ss/formula/functions/TestWeekNumFunc.java
+++ b/poi/src/test/java/org/apache/poi/ss/formula/functions/TestWeekNumFunc.java
@@ -24,6 +24,7 @@ import org.apache.poi.util.StringUtil;
 import org.junit.jupiter.api.Test;
 
 import java.time.LocalDate;
+import java.time.LocalDateTime;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.fail;
@@ -51,6 +52,19 @@ class TestWeekNumFunc {
         assertEvaluateEquals(10.0, date, 21);
     }
 
+    @Test
+    void testBug65606() {
+        double date = DateUtil.getExcelDate(LocalDate.parse("2021-09-27"));
+        assertEvaluateEquals(40.0, date);
+        assertEvaluateEquals(39.0, date, 21);
+    }
+
+    @Test
+    void testDateTime() {
+        double date = DateUtil.getExcelDate(LocalDateTime.parse("2021-09-27T09:45:00"));
+        assertEvaluateEquals(40.0, date);
+    }
+
     @Test
     void testEvaluateInvalid() {
         assertEvaluateEquals("no args",       ErrorEval.VALUE_INVALID);