Simplify some date handling edge-cases.

MarkPflug · Aug 16, 2023 · f45f4f4 · f45f4f4
1 parent 318192c
commit f45f4f4
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Showing 2 changed files with 8 additions and 8 deletions.
diff --git a/source/Sylvan.Data.Excel/ExcelDataReader.cs b/source/Sylvan.Data.Excel/ExcelDataReader.cs
@@ -47,7 +47,7 @@ public abstract partial class ExcelDataReader : DbDataReader, IDisposable, IDbCo
 	private protected int rowCount;
 	private protected int rowFieldCount;
 
-	static readonly DateTime Epoch1900 = new DateTime(1899, 12, 31);
+	static readonly DateTime Epoch1900 = new DateTime(1899, 12, 30);
 	static readonly DateTime Epoch1904 = new DateTime(1904, 1, 1);
 
 	private protected DateMode dateMode;
@@ -704,6 +704,7 @@ static internal bool TryGetDate(ExcelFormat fmt, double value, DateMode mode, ou
 	{
 		dt = DateTime.MinValue;
 		DateTime epoch = Epoch1904;
+		// Excel doesn't render negative values as dates.
 		if (value < 0.0)
 			return false;
 		if (mode == DateMode.Mode1900)
@@ -727,12 +728,11 @@ static internal bool TryGetDate(ExcelFormat fmt, double value, DateMode mode, ou
 				if (value >= 60d)
 				{
 					// 1900 wasn't a leapyear, but Excel thinks it was
+					// values in this range are in-expressible as .NET dates
+					// Excel renders it as 1900-2-29 (not a real day)
 					return false;
 				}
-			}
-			else
-			{
-				value -= 1;
+				value += 1;
 			}
 		}
 		dt = epoch.AddDays(value);

diff --git a/source/Sylvan.Data.Excel/Xlsx/XlsxDataWriter+FieldWriter.cs b/source/Sylvan.Data.Excel/Xlsx/XlsxDataWriter+FieldWriter.cs
@@ -7,7 +7,7 @@ namespace Sylvan.Data.Excel.Xlsx;
 
 partial class XlsxDataWriter
 {
-	static DateTime Epoch = new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Unspecified);
+	static DateTime Epoch = new DateTime(1899, 12, 30, 0, 0, 0, DateTimeKind.Unspecified);
 
 	sealed class Context
 	{
@@ -228,7 +228,7 @@ public static void WriteDateTime(Context c, DateTime value)
 			var w = c.xw;
 			w.Write("<c s=\"1\"><v>");
 
-			var val = (value - Epoch).TotalDays + 2;
+			var val = (value - Epoch).TotalDays;
 #if SPAN
 			var scratch = c.GetCharBuffer();
 			if (val.TryFormat(scratch.AsSpan(), out var sl, default, CultureInfo.InvariantCulture))
@@ -288,7 +288,7 @@ public static void WriteDateOnly(Context c, DateOnly value)
 			var w = c.xw;
 			w.Write("<c s=\"2\"><v>");
 
-			var val = (value.ToDateTime(Midnight) - Epoch).TotalDays + 2;
+			var val = (value.ToDateTime(Midnight) - Epoch).TotalDays;
 
 			var scratch = c.GetCharBuffer();
 			if (val.TryFormat(scratch.AsSpan(), out var sl, default, CultureInfo.InvariantCulture))