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))