Issue #48 implemented Days function

src/EPPlus/FormulaParsing/Excel/Functions/BuiltInFunctions.cs

@@ -182,6 +182,7 @@ public BuiltInFunctions()
             Functions["second"] = new Second();
             Functions["weeknum"] = new Weeknum();
             Functions["weekday"] = new Weekday();
+            Functions["days"] = new Days();
             Functions["days360"] = new Days360();
             Functions["yearfrac"] = new Yearfrac();
             Functions["edate"] = new Edate();

src/EPPlus/FormulaParsing/Excel/Functions/DateTime/Days.cs

@@ -0,0 +1,33 @@
+/*************************************************************************************************
+  Required Notice: Copyright (C) EPPlus Software AB. 
+  This software is licensed under PolyForm Noncommercial License 1.0.0 
+  and may only be used for noncommercial purposes 
+  https://polyformproject.org/licenses/noncommercial/1.0.0/
+
+  A commercial license to use this software can be purchased at https://epplussoftware.com
+ *************************************************************************************************
+  Date               Author                       Change
+ *************************************************************************************************
+  01/27/2020         EPPlus Software AB       Initial release EPPlus 5
+ *************************************************************************************************/
+using OfficeOpenXml.FormulaParsing.ExpressionGraph;
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace OfficeOpenXml.FormulaParsing.Excel.Functions.DateTime
+{
+    public class Days : ExcelFunction
+    {
+        public override CompileResult Execute(IEnumerable<FunctionArgument> arguments, ParsingContext context)
+        {
+            ValidateArguments(arguments, 2);
+            ValidateArguments(arguments, 2);
+            var numDate1 = ArgToDecimal(arguments, 0);
+            var numDate2 = ArgToDecimal(arguments, 1);
+            var endDate = System.DateTime.FromOADate(numDate1);
+            var startDate = System.DateTime.FromOADate(numDate2);
+            return CreateResult(endDate.Subtract(startDate).TotalDays, DataType.Date);
+        }
+    }
+}

src/EPPlus/FormulaParsing/Excel/Functions/Math/Odd.cs

@@ -27,7 +27,7 @@ public override CompileResult Execute(IEnumerable<FunctionArgument> arguments, P
             var arg = arguments.ElementAt(0).Value;
             if (!IsNumeric(arg)) return new CompileResult(eErrorType.Value);
             var number = ConvertUtil.GetValueDouble(arg);
-            if(number % 1 > 0)
+            if(number % 1 != 0)
             {
                 if (number >= 0)
                 {

src/EPPlusTest/FormulaParsing/Excel/Functions/DateTimeFunctionsTests.cs

@@ -125,6 +125,26 @@ public void DayShouldReturnMonthOfYearWithStringParam()
             Assert.AreEqual(12, result.Result);
         }
 
+        [TestMethod]
+        public void DaysShouldReturnCorrectResultWithDateTimeTypes()
+        {
+            var d1 = new DateTime(2015, 1, 1);
+            var d2 = new DateTime(2015, 2, 2);
+            var func = new Days();
+            var result = func.Execute(FunctionsHelper.CreateArgs(d2, d1), _parsingContext);
+            Assert.AreEqual(32d, result.Result);
+        }
+
+        [TestMethod]
+        public void DaysShouldReturnCorrectResultWithDatesAsStrings()
+        {
+            var d1 = new DateTime(2015, 1, 1).ToString("yyyy-MM-dd");
+            var d2 = new DateTime(2015, 2, 2).ToString("yyyy-MM-dd");
+            var func = new Days();
+            var result = func.Execute(FunctionsHelper.CreateArgs(d2, d1), _parsingContext);
+            Assert.AreEqual(32d, result.Result);
+        }
+
         [TestMethod]
         public void MonthShouldReturnMonthOfYear()
         {

src/EPPlusTest/FormulaParsing/IntegrationTests/BuiltInFunctions/DateAndTimeFunctionsTests.cs

@@ -158,6 +158,13 @@ public void HourShouldReturnCorrectResultWhenParsingString()
             Assert.AreEqual(10, result);
         }
 
+        [TestMethod]
+        public void DaysShouldReturnCorrectResult()
+        {
+            var result = _parser.Parse("Days(Date(2015, 2, 2), Date(2015, 1, 1))");
+            Assert.AreEqual(32d, result);
+        }
+
         [TestMethod]
         public void Day360ShouldReturnCorrectResult()
         {