Read built-in number format 17 (mmm-yy) as DateTime

src/MiniExcel/OpenXml/ExcelOpenXmlStyles.cs

@@ -154,7 +154,7 @@ public object ConvertValueByStyleFormat(int index, object value)
         { 14, new NumberFormatString("d/m/yyyy",typeof(DateTime?)) },
         { 15, new NumberFormatString("d-mmm-yy",typeof(DateTime?)) },
         { 16, new NumberFormatString("d-mmm",typeof(DateTime?)) },
-        { 17, new NumberFormatString("mmm-yy",typeof(TimeSpan)) },
+        { 17, new NumberFormatString("mmm-yy",typeof(DateTime?)) },
         { 18, new NumberFormatString("h:mm AM/PM",typeof(TimeSpan?)) },
         { 19, new NumberFormatString("h:mm:ss AM/PM",typeof(TimeSpan?)) },
         { 20, new NumberFormatString("h:mm",typeof(TimeSpan?)) },

tests/MiniExcelTests/MiniExcelIssueTests.cs

@@ -486,7 +486,7 @@
         var sheets = MiniExcel.GetSheetNames(path);
         foreach (var sheetName in sheets)
         {
             var dt = MiniExcel.QueryAsDataTable(path, useHeaderRow: true, sheetName: sheetName, configuration: config);
         }
     }
 
@@ -4919,4 +4919,37 @@
         Assert.Equal(7.4, result[1].Altitude);
         Assert.Equal(8.6, result[2].Altitude);
     }
+
+    // https://github.com/mini-software/MiniExcel/issues/1034
+    [Theory]
+    [InlineData(14)]
+    [InlineData(15)]
+    [InlineData(16)]
+    [InlineData(17)]
+    [InlineData(22)]
+    public void QueryReadsBuiltInDateFormatsAsDateTime(int numFmtId)
+    {
+        using var path = AutoDeletingPath.Create();
+        MiniExcel.SaveAs(path.ToString(), new[] { new { Value = "control" } });
+        Helpers.ReplaceStylesXml(path.ToString(), $"""
+            <?xml version="1.0" encoding="utf-8"?>
+            <styleSheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main">
+                <cellXfs count="2">
+                    <xf numFmtId="0" fontId="0" fillId="0" borderId="0" xfId="0" />
+                    <xf numFmtId="{numFmtId}" fontId="0" fillId="0" borderId="0" xfId="0" applyNumberFormat="1" />
+                </cellXfs>
+            </styleSheet>
+            """);
+        Helpers.ReplaceFirstSheetXml(path.ToString(), """
+            <?xml version="1.0" encoding="utf-8"?>
+            <worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main">
+                <dimension ref="A1" />
+                <sheetData><row r="1"><c r="A1" s="1"><v>46327</v></c></row></sheetData>
+            </worksheet>
+            """);
+
+        var row = MiniExcel.Query(path.ToString()).Single();
+
+        Assert.Equal(new DateTime(2026, 11, 1), (object)row.A);
+    }
 }

tests/MiniExcelTests/Utils/Helpers.cs

@@ -91,6 +91,21 @@ internal static void ReplaceFirstSheetXml(string path, string worksheetXml)
         writer.Write(worksheetXml);
     }
 
+    internal static void ReplaceStylesXml(string path, string stylesXml)
+    {
+        using var stream = File.Open(path, FileMode.Open, FileAccess.ReadWrite);
+        using var archive = new ZipArchive(stream, ZipArchiveMode.Update, false, Encoding.UTF8);
+        var styles = archive.Entries.Single(w =>
+            w.FullName.Equals("xl/styles.xml", StringComparison.OrdinalIgnoreCase) ||
+            w.FullName.Equals("/xl/styles.xml", StringComparison.OrdinalIgnoreCase));
+        var entryName = styles.FullName;
+        styles.Delete();
+
+        var replacement = archive.CreateEntry(entryName);
+        using var writer = new StreamWriter(replacement.Open(), new UTF8Encoding(false));
+        writer.Write(stylesXml);
+    }
+
     internal static string? GetFirstSheetDimensionRefValue(string path)
     {
         var ns = new XmlNamespaceManager(new NameTable());