diff --git a/src/changelog/3.5.1/334-rollingfileappender-backup-index.xml b/src/changelog/3.5.1/334-rollingfileappender-backup-index.xml
new file mode 100644
index 00000000..76148837
--- /dev/null
+++ b/src/changelog/3.5.1/334-rollingfileappender-backup-index.xml
@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xmlns="https://logging.apache.org/xml/ns"
+       xsi:schemaLocation="https://logging.apache.org/xml/ns https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="fixed">
+  <issue id="334" link="https://github.com/apache/logging-log4net/pull/334"/>
+  <description format="asciidoc">Count only the current date's backups when `RollingFileAppender`
+  starts. A date pattern without separators (`.yyyyMMdd`) no longer counts as a backup number, and
+  backups of earlier dates no longer raise the count (reported by @Snotface, fixed by @FreeAndNil)</description>
+</entry>
diff --git a/src/log4net.Tests/Appender/RollingFileAppenderTest.cs b/src/log4net.Tests/Appender/RollingFileAppenderTest.cs
index f67c8818..8ef01257 100644
--- a/src/log4net.Tests/Appender/RollingFileAppenderTest.cs
+++ b/src/log4net.Tests/Appender/RollingFileAppenderTest.cs
@@ -1664,6 +1664,163 @@ public void TestInitializeRollBackups4()
     VerifyInitializeRollBackups(8, maxRollBackups);
   }
 
+  /// <summary>
+  /// Tests that a date roll file using a date pattern without "-" (yyyyMMdd)
+  /// is not mistaken for a backup index, whatever the backup limit
+  /// </summary>
+  [TestCase(5, 1)]
+  [TestCase(5, -1)]
+  [TestCase(-1, 1)]
+  [TestCase(-1, -1)]
+  public void TestInitializeRollBackupsIgnoresDateWithoutDashes(int maxSizeRollBackups,
+    int countDirection)
+  {
+    const string baseFile = "LogFile.log";
+    List<string> files = ["LogFile.log", "LogFile.log.20261003", "LogFile.log.1", "LogFile.log.2"];
+
+    RollingFileAppender rfa = new()
+    {
+      RollingStyle = RollingFileAppender.RollingMode.Composite,
+      DatePattern = ".yyyyMMdd",
+      CountDirection = countDirection,
+      MaxSizeRollBackups = maxSizeRollBackups,
+      CurrentSizeRollBackups = 0
+    };
+    rfa.InitializeRollBackups(baseFile, files);
+
+    Assert.That(rfa.CurrentSizeRollBackups, Is.EqualTo(2));
+  }
+
+  /// <summary>
+  /// Tests that counting up continues from the highest index, even when
+  /// the oldest backups were deleted and the indexes passed the backup limit
+  /// </summary>
+  [Test]
+  public void TestInitializeRollBackupsCountsUpPastLimit()
+  {
+    const string baseFile = "LogFile.log";
+    List<string> files = ["LogFile.log", "LogFile.log.5", "LogFile.log.6", "LogFile.log.7"];
+
+    RollingFileAppender rfa = new()
+    {
+      RollingStyle = RollingFileAppender.RollingMode.Composite,
+      CountDirection = 1,
+      MaxSizeRollBackups = 3,
+      CurrentSizeRollBackups = 0
+    };
+    rfa.InitializeRollBackups(baseFile, files);
+
+    Assert.That(rfa.CurrentSizeRollBackups, Is.EqualTo(7));
+  }
+
+  /// <summary>
+  /// Tests that backups already rolled to an earlier date are not counted
+  /// as backups of the current date
+  /// </summary>
+  [Test]
+  public void TestInitializeRollBackupsIgnoresOtherDatePeriods()
+  {
+    const string baseFile = "LogFile.log";
+    List<string> files =
+    [
+      "LogFile.log",
+      "LogFile.log.20261003",
+      "LogFile.log.20261003.1",
+      "LogFile.log.20261003.2",
+      "LogFile.log.20261003.3",
+      "LogFile.log.1",
+    ];
+
+    RollingFileAppender rfa = new()
+    {
+      RollingStyle = RollingFileAppender.RollingMode.Composite,
+      DatePattern = ".yyyyMMdd",
+      MaxSizeRollBackups = -1,
+      CurrentSizeRollBackups = 0
+    };
+    rfa.InitializeRollBackups(baseFile, files);
+
+    Assert.That(rfa.CurrentSizeRollBackups, Is.EqualTo(1));
+  }
+
+  /// <summary>
+  /// Tests that backups of the current date are found when the date pattern
+  /// supplies the extension and the extension is preserved
+  /// </summary>
+  [Test]
+  public void TestInitializeRollBackupsDatePatternWithExtension()
+  {
+    const string baseFile = "log4net";
+    List<string> files =
+    [
+      "log4net.2026-10-04.log",
+      "log4net.2026-10-04.1.log",
+      "log4net.2026-10-04.2.log",
+      "log4net.2026-10-03.log",
+      "log4net.2026-10-03.5.log",
+    ];
+
+    RollingFileAppender rfa = new()
+    {
+      RollingStyle = RollingFileAppender.RollingMode.Composite,
+      DatePattern = ".yyyy-MM-dd'.log'",
+      DateTimeStrategy = new Integration.MockDateTime(new DateTime(2026, 10, 4, 9, 0, 0)),
+      StaticLogFileName = false,
+      PreserveLogFileNameExtension = true,
+      MaxSizeRollBackups = -1,
+      CurrentSizeRollBackups = 0
+    };
+    rfa.InitializeRollBackups(baseFile, files);
+
+    Assert.That(rfa.CurrentSizeRollBackups, Is.EqualTo(2));
+  }
+
+  /// <summary>
+  /// Tests that the current date's file of a non-static name is not mistaken for a backup index
+  /// when the date pattern has no separators
+  /// </summary>
+  [TestCase("log.txt", ".yyyyMMdd", false, "log.txt.20261004", "log.txt.20261004.1")]
+  [TestCase("log", ".yyyyMMdd'.log'", true, "log.20261004.log", "log.20261004.1.log")]
+  public void TestInitializeRollBackupsNotStaticDateWithoutDashes(string baseFile,
+    string datePattern, bool preserveExtension, string datedFile, string backupFile)
+  {
+    RollingFileAppender rfa = new()
+    {
+      RollingStyle = RollingFileAppender.RollingMode.Composite,
+      DatePattern = datePattern,
+      DateTimeStrategy = new Integration.MockDateTime(new DateTime(2026, 10, 4, 9, 0, 0)),
+      StaticLogFileName = false,
+      PreserveLogFileNameExtension = preserveExtension,
+      MaxSizeRollBackups = -1,
+      CurrentSizeRollBackups = 0
+    };
+    rfa.InitializeRollBackups(baseFile, [datedFile, backupFile]);
+
+    Assert.That(rfa.CurrentSizeRollBackups, Is.EqualTo(1));
+  }
+
+  /// <summary>
+  /// Tests that with a short date pattern the suffixes of a static file name count as backup
+  /// indexes, as a date like .10 cannot be told apart from backup 10
+  /// </summary>
+  [Test]
+  public void TestInitializeRollBackupsShortDatePatternCountsIndexes()
+  {
+    const string baseFile = "log.txt";
+    List<string> files = ["log.txt", .. Enumerable.Range(1, 10).Select(i => $"log.txt.{i}")];
+
+    RollingFileAppender rfa = new()
+    {
+      RollingStyle = RollingFileAppender.RollingMode.Date,
+      DatePattern = ".dd",
+      MaxSizeRollBackups = -1,
+      CurrentSizeRollBackups = 0
+    };
+    rfa.InitializeRollBackups(baseFile, files);
+
+    Assert.That(rfa.CurrentSizeRollBackups, Is.EqualTo(10));
+  }
+
   /// <summary>
   /// Ensures that no problems result from creating and then closing the appender
   /// when it has not also been initialized with ActivateOptions().
diff --git a/src/log4net/Appender/RollingFileAppender.cs b/src/log4net/Appender/RollingFileAppender.cs
index d6ad43f6..0cbdd144 100644
--- a/src/log4net/Appender/RollingFileAppender.cs
+++ b/src/log4net/Appender/RollingFileAppender.cs
@@ -879,29 +879,29 @@ private void InitializeFromOneFile(string baseFile, string curFileName)
     if (_rollDate && !StaticLogFileName)
     {
       string date = DateTimeStrategy.Now.ToString(DatePattern, DateTimeFormatInfo.InvariantInfo).ToLowerInvariant();
-      string prefix = (PreserveLogFileNameExtension
-        ? Path.GetFileNameWithoutExtension(baseFile) + date
-        : baseFile + date).ToLowerInvariant();
-      string suffix = PreserveLogFileNameExtension
-        ? Path.GetExtension(baseFile).ToLowerInvariant()
-        : "";
+      // the date pattern may supply the extension, so take it from the dated name
+      string dated = Path.GetFileName(CombinePath(baseFile, date));
+      string suffix = PreserveLogFileNameExtension ? Path.GetExtension(dated) : "";
+      string prefix = dated.Substring(0, dated.Length - suffix.Length);
       string curFileNameWithoutDir = Path.GetFileName(curFileName);
       if (!curFileNameWithoutDir.StartsWith(prefix) || !curFileNameWithoutDir.EndsWith(suffix))
       {
         LogLog.Debug(_declaringType, $"Ignoring file [{curFileName}] because it is from a different date period");
         return;
       }
+
+      // the dated file itself has no index, and a date like .yyyyMMdd would parse as one
+      if (curFileNameWithoutDir.Length <= dated.Length)
+      {
+        return;
+      }
     }
 
     try
     {
       // Bump the counter up to the highest count seen so far
-      int backup = GetBackupIndex(curFileName);
+      int backup = GetBackupIndex(baseFile, curFileName);
 
-      // caution: we might get a false positive when certain
-      // date patterns such as yyyyMMdd are used...those are
-      // valid number but aren't the kind of back up index
-      // we're looking for
       if (backup > CurrentSizeRollBackups)
       {
         if (0 == MaxSizeRollBackups)
@@ -945,10 +945,7 @@ private void InitializeFromOneFile(string baseFile, string curFileName)
   /// Attempts to extract a number from the end of the file name that indicates
   /// the number of the times the file has been rolled over.
   /// </summary>
-  /// <remarks>
-  /// Certain date pattern extensions like yyyyMMdd will be parsed as valid backup indexes.
-  /// </remarks>
-  private int GetBackupIndex(string curFileName)
+  private int GetBackupIndex(string baseFile, string curFileName)
   {
     int result = -1;
     string fileName = curFileName;
@@ -956,11 +953,20 @@ private int GetBackupIndex(string curFileName)
     if (PreserveLogFileNameExtension)
     {
       fileName = Path.GetFileNameWithoutExtension(fileName);
+      baseFile = Path.GetFileNameWithoutExtension(baseFile);
     }
 
     int index = fileName.LastIndexOf(".", StringComparison.Ordinal);
     if (index > 0)
     {
+      // with a static name, log.txt.20261003.1 is a backup of an earlier date
+      if (StaticLogFileName
+        && (!string.Equals(fileName.Substring(0, index), baseFile, StringComparison.Ordinal)
+          || IsDateSuffix(fileName.Substring(index))))
+      {
+        return -1;
+      }
+
       // if the "yyyy-MM-dd" component of file.log.yyyy-MM-dd is passed to TryParse
       // it will gracefully fail and return backUpIndex will be 0
       _ = SystemInfo.TryParse(fileName.Substring(index + 1), out result);
@@ -969,6 +975,16 @@ private int GetBackupIndex(string curFileName)
     return result;
   }
 
+  /// <summary>
+  /// Determines whether <paramref name="suffix"/> is a date longer than four characters after
+  /// the dot, such as <c>.yyyyMMdd</c>. Shorter dates (<c>.dd</c> writes <c>.10</c>) cannot be
+  /// told from a backup index and keep counting as one.
+  /// </summary>
+  private bool IsDateSuffix(string suffix)
+    => _rollDate && DatePattern is not null && suffix.Length > 5
+      && DateTime.TryParseExact(suffix, DatePattern, DateTimeFormatInfo.InvariantInfo,
+        DateTimeStyles.None, out _);
+
   /// <summary>
   /// Takes a list of files and a base file name, and looks for 'incremented' versions of the base file.
   /// Bumps the max count up to the highest count seen.