diff --git a/src/Simplexcel.TestApp/Program.cs b/src/Simplexcel.TestApp/Program.cs
index e48ff8e..f5f0399 100644
--- a/src/Simplexcel.TestApp/Program.cs
+++ b/src/Simplexcel.TestApp/Program.cs
@@ -71,33 +71,35 @@ static void Main()
             sheet.Cells[0, 6] = "👪";
             sheet.Cells[0, 7] = "👨‍👩‍👧‍👦";
 
-            sheet.Cells["D4"] = DateTime.Now;
-            sheet.Cells["D5"] = new Cell(CellType.Date, DateTime.Now, BuiltInCellFormat.DateOnly);
-            sheet.Cells["D6"] = new Cell(CellType.Date, DateTime.Now, BuiltInCellFormat.TimeOnly);
-            sheet.Cells["D7"] = long.MaxValue;
-            sheet.Cells["D8"] = long.MinValue;
-            sheet.Cells["D9"] = decimal.MaxValue;
-            sheet.Cells["D10"] = decimal.MinValue;
-
-            sheet.Cells["D11"] = 9999999999L;
-            sheet.Cells["D12"] = 99999999999L;
-            sheet.Cells["D13"] = 100000000000L;
-            sheet.Cells["D14"] = 100000000001L;
-            sheet.Cells["D15"] = 1000000000000L;
-            sheet.Cells["D16"] = 1000000000001L;
-            sheet.Cells["D17"] = Cell.LargeNumberPositiveLimit;
-            sheet.Cells["D18"] = Cell.LargeNumberPositiveLimit + 1;
-            sheet.Cells["D19"] = Cell.LargeNumberPositiveLimit - 1;
-
-            sheet.Cells["D20"] = -9999999999L;
-            sheet.Cells["D21"] = -99999999999L;
-            sheet.Cells["D22"] = -100000000000L;
-            sheet.Cells["D23"] = -100000000001L;
-            sheet.Cells["D24"] = -1000000000000L;
-            sheet.Cells["D25"] = -1000000000001L;
-            sheet.Cells["D26"] = Cell.LargeNumberNegativeLimit;
-            sheet.Cells["D27"] = Cell.LargeNumberNegativeLimit + 1;
-            sheet.Cells["D28"] = Cell.LargeNumberNegativeLimit - 1;
+            var dateTime = new DateTime(2022, 4, 26, 13, 14, 15);
+            sheet.Cells["D4"] = dateTime;
+            sheet.Cells["D5"] = new Cell(CellType.Date, dateTime, BuiltInCellFormat.DateOnly);
+            sheet.Cells["D6"] = new Cell(CellType.Date, dateTime, BuiltInCellFormat.TimeOnly);
+            sheet.Cells["D7"] = new Cell(CellType.Date, dateTime, "yyyy\"_\"mm\"_\"dd\"_\"hh\"_\"mm\"_\"ss");
+            sheet.Cells["D8"] = long.MaxValue;
+            sheet.Cells["D9"] = long.MinValue;
+            sheet.Cells["D10"] = decimal.MaxValue;
+            sheet.Cells["D11"] = decimal.MinValue;
+
+            sheet.Cells["D12"] = 9999999999L;
+            sheet.Cells["D13"] = 99999999999L;
+            sheet.Cells["D14"] = 100000000000L;
+            sheet.Cells["D15"] = 100000000001L;
+            sheet.Cells["D16"] = 1000000000000L;
+            sheet.Cells["D17"] = 1000000000001L;
+            sheet.Cells["D18"] = Cell.LargeNumberPositiveLimit;
+            sheet.Cells["D19"] = Cell.LargeNumberPositiveLimit + 1;
+            sheet.Cells["D20"] = Cell.LargeNumberPositiveLimit - 1;
+
+            sheet.Cells["D21"] = -9999999999L;
+            sheet.Cells["D22"] = -99999999999L;
+            sheet.Cells["D23"] = -100000000000L;
+            sheet.Cells["D24"] = -100000000001L;
+            sheet.Cells["D25"] = -1000000000000L;
+            sheet.Cells["D26"] = -1000000000001L;
+            sheet.Cells["D27"] = Cell.LargeNumberNegativeLimit;
+            sheet.Cells["D28"] = Cell.LargeNumberNegativeLimit + 1;
+            sheet.Cells["D29"] = Cell.LargeNumberNegativeLimit - 1;
             sheet.LargeNumberHandlingMode = LargeNumberHandlingMode.StoreAsText;
 
             sheet.Cells["C5"] = "ThinDiagonalCrosshatch";
diff --git a/src/Simplexcel/Cells/BuiltInCellFormat.cs b/src/Simplexcel/Cells/BuiltInCellFormat.cs
index d4d4dba..3c0ca72 100644
--- a/src/Simplexcel/Cells/BuiltInCellFormat.cs
+++ b/src/Simplexcel/Cells/BuiltInCellFormat.cs
@@ -36,14 +36,14 @@ public static class BuiltInCellFormat
         public const string Text = "@";
 
         /// <summary>
-        /// m/d/yyyy h:mm
+        /// m/d/yy h:mm
         /// </summary>
-        public const string DateAndTime = "m/d/yyyy h:mm";
+        public const string DateAndTime = "m/d/yy h:mm";
 
         /// <summary>
-        /// m/d/yyyy
+        /// mm-dd-yy
         /// </summary>
-        public const string DateOnly = "m/d/yyyy";
+        public const string DateOnly = "mm-dd-yy";
 
         /// <summary>
         /// h:mm
diff --git a/src/Simplexcel/XlsxInternal/StyleWriter.cs b/src/Simplexcel/XlsxInternal/StyleWriter.cs
index 4f744e5..dc8e674 100644
--- a/src/Simplexcel/XlsxInternal/StyleWriter.cs
+++ b/src/Simplexcel/XlsxInternal/StyleWriter.cs
@@ -9,6 +9,41 @@ internal static class StyleWriter
         // There are up to 164 built in number formats (0-163), all else are custom (164 and above)
         private const int CustomFormatIndex = 164;
 
+        /// <summary>
+        /// Standard format codes as defined in ECMA-376, 3rd Edition, Part 1, 18.8.30 numFmt (Number Format)
+        /// </summary>
+        private static Dictionary<string, int> StandardFormatIds = new Dictionary<string, int>
+        {
+            ["General"] = 0,
+            ["0"] = 1,
+            ["0.00"] = 2,
+            ["#,##0"] = 3,
+            ["#,##0.00"] = 4,
+            ["0%"] = 9,
+            ["0.00%"] = 10,
+            ["0.00E+00"] = 11,
+            ["# ?/?"] = 12,
+            ["# ??/??"] = 13,
+            ["mm-dd-yy"] = 14,
+            ["d-mmm-yy"] = 15,
+            ["d-mmm"] = 16,
+            ["mmm-yy"] = 17,
+            ["h:mm AM/PM"] = 18,
+            ["h:mm:ss AM/PM"] = 19,
+            ["h:mm"] = 20,
+            ["h:mm:ss"] = 21,
+            ["m/d/yy h:mm"] = 22,
+            ["#,##0 ;(#,##0)"] = 37,
+            ["#,##0 ;[Red](#,##0)"] = 38,
+            ["#,##0.00;(#,##0.00)"] = 39,
+            ["#,##0.00;[Red](#,##0.00)"] = 40,
+            ["mm:ss"] = 45,
+            ["[h]:mm:ss"] = 46,
+            ["mmss.0"] = 47,
+            ["##0.0E+0"] = 48,
+            ["@"] = 49,
+        };
+
         /// <summary>
         /// Create a styles.xml file
         /// </summary>
@@ -29,7 +64,7 @@ internal static XmlFile CreateStyleXml(IList<XlsxCellStyle> styles)
                     uniqueBorders.Add(style.Border);
                 }
 
-                if (!numberFormats.Contains(style.Format))
+                if (!numberFormats.Contains(style.Format) && !StandardFormatIds.ContainsKey(style.Format))
                 {
                     numberFormats.Add(style.Format);
                 }
@@ -79,7 +114,7 @@ private static void StyleAddCellXfsElement(XDocument doc, IList<XlsxCellStyle> s
 
             foreach (var style in styles)
             {
-                var numFmtId = numberFormats.IndexOf(style.Format) + CustomFormatIndex;
+                var numFmtId = StandardFormatIds.TryGetValue(style.Format, out var standardNumFmtId) ? standardNumFmtId : numberFormats.IndexOf(style.Format) + CustomFormatIndex;
                 var fontId = fontInfos.IndexOf(style.Font);
                 var fillId = fills.IndexOf(style.Fill);
                 var borderId = uniqueBorders.IndexOf(style.Border);