Index: src/java/org/apache/poi/ss/formula/functions/Days.java
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--- src/java/org/apache/poi/ss/formula/functions/Days.java	(revision 0)
+++ src/java/org/apache/poi/ss/formula/functions/Days.java	(working copy)
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+/* ====================================================================
+   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 java.util.Calendar;
+
+import org.apache.poi.ss.formula.eval.EvaluationException;
+import org.apache.poi.ss.formula.eval.NumberEval;
+import org.apache.poi.ss.formula.eval.OperandResolver;
+import org.apache.poi.ss.formula.eval.ValueEval;
+import org.apache.poi.ss.usermodel.DateUtil;
+import org.apache.poi.util.LocaleUtil;
+
+/**
+ * <p>Calculates the number of days between two dates based on a real year
+ * , which is used in some accounting calculations. Use
+ * this function to help compute payments if your accounting system is based on
+ * twelve 30-day months.</p>
+ *
+ * <p> 
+ * {@code DAYS(end_date,start_date)}
+ * 
+ * <ul>
+ * <li>Start_date, end_date (required):<br/>
+ * The two dates between which you want to know the number of days.<br/>
+ * If start_date occurs after end_date, the DAYS function returns a negative number.</li>
+ * 
+ * </ul>
+ * </p>
+ * 
+ * @see <a href="https://support.office.com/en-us/article/DAYS-function-57740535-D549-4395-8728-0F07BFF0B9DF">DAYS function - Microsoft Office</a>
+ */
+public class Days extends Fixed2ArgFunction {
+
+    private final static long MILLIS_PER_DAY = 1000*60*60*24;
+
+    public ValueEval evaluate(int srcRowIndex, int srcColumnIndex, ValueEval arg0, ValueEval arg1) {
+        double result;
+        try {
+            double d0 = NumericFunction.singleOperandEvaluate(arg0, srcRowIndex, srcColumnIndex);
+            double d1 = NumericFunction.singleOperandEvaluate(arg1, srcRowIndex, srcColumnIndex);
+            result = evaluate(d0, d1);
+        } catch (EvaluationException e) {
+            return e.getErrorEval();
+        }
+        return new NumberEval(result);
+    }
+
+    private static double evaluate(double d0, double d1) {
+        Calendar endDate = getDate(d0);
+        Calendar startDate = getDate(d1);
+
+        return daysBetween(startDate.getTimeInMillis(), endDate.getTimeInMillis());
+    }
+
+    private static Calendar getDate(double date) {
+        Calendar processedDate = LocaleUtil.getLocaleCalendar();
+        processedDate.setTime(DateUtil.getJavaDate(date, false));
+        return processedDate;
+    }
+
+    /** Replace me with org.joda.time.Days.daysBetween */
+    private static double daysBetween(long start, long end) {
+        return (end - start) / MILLIS_PER_DAY;
+    }
+}

Property changes on: src/java/org/apache/poi/ss/formula/functions/Days.java
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