diff --git a/ClosedXML/Excel/CalcEngine/Functions/Tally.cs b/ClosedXML/Excel/CalcEngine/Functions/Tally.cs
index 5cf212e..8796b84 100644
--- a/ClosedXML/Excel/CalcEngine/Functions/Tally.cs
+++ b/ClosedXML/Excel/CalcEngine/Functions/Tally.cs
@@ -13,7 +13,6 @@
 
         private double[] _numericValues;
 
-
         public Tally()
             : this(false)
         { }
@@ -84,7 +83,7 @@
                 return _list.Count(o => !Statistical.IsBlank(o));
         }
 
-        IEnumerable<double> NumericValuesEnumerable()
+        private IEnumerable<double> NumericValuesEnumerable()
         {
             foreach (var value in _list)
             {
@@ -92,14 +91,14 @@
                 var vEnumerable = value as IEnumerable;
                 if (vEnumerable == null)
                 {
-                    if (double.TryParse(value.ToString(), out tmp))
+                    if (TryParseToDouble(value, out tmp))
                         yield return tmp;
                 }
                 else
                 {
                     foreach (var v in vEnumerable)
                     {
-                        if (double.TryParse(v.ToString(), out tmp))
+                        if (TryParseToDouble(v, out tmp))
                             yield return tmp;
                         break;
                     }
@@ -107,7 +106,25 @@
             }
         }
 
-        double[] NumericValuesInternal()
+        private bool TryParseToDouble(object value, out double d)
+        {
+            if (value.IsNumber())
+            {
+                d = Convert.ToDouble(value);
+                return true;
+            }
+            else if (value is DateTime)
+            {
+                d = Convert.ToDouble(((DateTime)value).ToOADate());
+                return true;
+            }
+            else
+            {
+                return double.TryParse(value.ToString(), out d);
+            }
+        }
+
+        private double[] NumericValuesInternal()
             => LazyInitializer.EnsureInitialized(ref _numericValues, () => NumericValuesEnumerable().ToArray());
 
         public IEnumerable<double> NumericValues()
@@ -144,7 +161,7 @@
 
         public double Range() => Max() - Min();
 
-        static double Sum2(IEnumerable<double> nums)
+        private static double Sum2(IEnumerable<double> nums)
         {
             return nums.Sum(d => d * d);
         }
diff --git a/ClosedXML_Tests/Excel/CalcEngine/FunctionsTests.cs b/ClosedXML_Tests/Excel/CalcEngine/FunctionsTests.cs
index 3ad7f8e..a4435ef 100644
--- a/ClosedXML_Tests/Excel/CalcEngine/FunctionsTests.cs
+++ b/ClosedXML_Tests/Excel/CalcEngine/FunctionsTests.cs
@@ -379,7 +379,7 @@
 
             actual = XLWorkbook.EvaluateExpr("Round(-50.55, -2)");
             Assert.AreEqual(-100.0, actual);
-            
+
             actual = XLWorkbook.EvaluateExpr("ROUND(59 * 0.535, 2)"); // (59 * 0.535) = 31.565
             Assert.AreEqual(31.57, actual);
 
@@ -574,6 +574,22 @@
         }
 
         [Test]
+        public void SumDateTimeAndNumber()
+        {
+            using (var wb = new XLWorkbook())
+            {
+                var ws = wb.AddWorksheet("Sheet1");
+                ws.Cell("A1").Value = 1;
+                ws.Cell("A2").Value = new DateTime(2018, 1, 1);
+                Assert.AreEqual(43102, ws.Evaluate("SUM(A1:A2)"));
+
+                ws.Cell("A1").Value = 2;
+                ws.Cell("A2").FormulaA1 = "DATE(2018,1,1)";
+                Assert.AreEqual(43103, ws.Evaluate("SUM(A1:A2)"));
+            }
+        }
+
+        [Test]
         public void SumSq()
         {
             Object actual;