--- poi/src/testcases/org/apache/poi/ss/formula/functions/TestWeekNumFunctionsFromSpreadsheet.java	(revision )
+++ poi/src/testcases/org/apache/poi/ss/formula/functions/TestWeekNumFunctionsFromSpreadsheet.java	(revision )
@@ -0,0 +1,31 @@ 
+/* ====================================================================
+   Licensed to the Apache Software Foundation (ASF) under one or more
+   contributor license agreements.  See the NOTICE file distributed with
+   this work for additional information regarding copyright ownership.
+   The ASF licenses this file to You under the Apache License, Version 2.0
+   (the "License"); you may not use this file except in compliance with
+   the License.  You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+==================================================================== */
+
+package org.apache.poi.ss.formula.functions;
+
+/**
+ * Tests WeekNum() as loaded from a test data spreadsheet.<p/>
+ *
+ * @author cedric dot walter @ gmail dot com
+ */
+public class TestWeekNumFunctionsFromSpreadsheet extends BaseTestFunctionsFromSpreadsheet {
+
+    @Override
+    protected String getFilename() {
+        return "WeekNumFunctionTestCaseData.xls";
+    }
+}
--- poi/src/java/org/apache/poi/ss/formula/atp/AnalysisToolPak.java	(revision 1490713)
+++ poi/src/java/org/apache/poi/ss/formula/atp/AnalysisToolPak.java	(revision )
@@ -158,7 +158,7 @@ 
         r(m, "TBILLEQ", null);
         r(m, "TBILLPRICE", null);
         r(m, "TBILLYIELD", null);
-        r(m, "WEEKNUM", null);
+        r(m, "WEEKNUM", WeekNum.instance);
         r(m, "WORKDAY", WorkdayFunction.instance);
         r(m, "XIRR", null);
         r(m, "XNPV", null);
--- poi/src/java/org/apache/poi/ss/formula/functions/WeekNum.java	(revision )
+++ poi/src/java/org/apache/poi/ss/formula/functions/WeekNum.java	(revision )
@@ -0,0 +1,88 @@ 
+/* ====================================================================
+   Licensed to the Apache Software Foundation (ASF) under one or more
+   contributor license agreements.  See the NOTICE file distributed with
+   this work for additional information regarding copyright ownership.
+   The ASF licenses this file to You under the Apache License, Version 2.0
+   (the "License"); you may not use this file except in compliance with
+   the License.  You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+==================================================================== */
+
+package org.apache.poi.ss.formula.functions;
+
+import org.apache.poi.ss.formula.OperationEvaluationContext;
+import org.apache.poi.ss.formula.eval.*;
+import org.apache.poi.ss.usermodel.DateUtil;
+
+import java.util.Calendar;
+import java.util.GregorianCalendar;
+
+/**
+ * Implementation for Excel WeekNum() function.<p/>
+ * <p/>
+ * <b>Syntax</b>:<br/> <b>WeekNum  </b>(<b>Serial_num</b>,<b>Return_type</b>)<br/>
+ * <p/>
+ * Returns a number that indicates where the week falls numerically within a year.
+ * <p/>
+ * <p/>
+ * Serial_num     is a date within the week. Dates should be entered by using the DATE function,
+ * or as results of other formulas or functions. For example, use DATE(2008,5,23)
+ * for the 23rd day of May, 2008. Problems can occur if dates are entered as text.
+ * Return_type     is a number that determines on which day the week begins. The default is 1.
+ * 1	Week begins on Sunday. Weekdays are numbered 1 through 7.
+ * 2	Week begins on Monday. Weekdays are numbered 1 through 7.
+ *
+ * @author cedric dot walter @ gmail dot com
+ */
+public class WeekNum extends Fixed2ArgFunction implements FreeRefFunction {
+
+    public static final FreeRefFunction instance = new WeekNum();
+
+    public ValueEval evaluate(int srcRowIndex, int srcColumnIndex, ValueEval serialNumVE, ValueEval returnTypeVE) {
+        double serialNum = 0.0;
+        try {
+            serialNum = NumericFunction.singleOperandEvaluate(serialNumVE, srcRowIndex, srcColumnIndex);
+        } catch (EvaluationException e) {
+            return ErrorEval.VALUE_INVALID;
+        }
+        Calendar serialNumCalendar = new GregorianCalendar();
+        serialNumCalendar.setTime(DateUtil.getJavaDate(serialNum, false));
+
+        int returnType = 0;
+        try {
+            ValueEval ve = OperandResolver.getSingleValue(returnTypeVE, srcRowIndex, srcColumnIndex);
+            returnType = OperandResolver.coerceValueToInt(ve);
+        } catch (EvaluationException e) {
+            return ErrorEval.NUM_ERROR;
+        }
+
+        if (returnType != 1 && returnType != 2) {
+            return ErrorEval.NUM_ERROR;
+        }
+
+        return new NumberEval(this.getWeekNo(serialNumCalendar, returnType));
+    }
+
+    public int getWeekNo(Calendar cal, int weekStartOn) {
+        if (weekStartOn == 1) {
+            cal.setFirstDayOfWeek(Calendar.SUNDAY);
+        } else {
+            cal.setFirstDayOfWeek(Calendar.MONDAY);
+        }
+        return cal.get(Calendar.WEEK_OF_YEAR);
+    }
+
+    public ValueEval evaluate(ValueEval[] args, OperationEvaluationContext ec) {
+        if (args.length == 2) {
+            return evaluate(ec.getRowIndex(), ec.getColumnIndex(), args[0], args[1]);
+        }
+        return ErrorEval.VALUE_INVALID;
+    }
+}