Fix "MMM.yy" date parsing in German culture. #114169

src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeParse.cs

@@ -3289,6 +3289,47 @@ private static bool ParseTimeZoneOffset(ref __DTString str, int len, scoped ref
             return true;
         }
 
+        /// Determines if a format string contains a day-of-month specifier ('d' or 'dd').
+        /// Properly handles quoted sections and escape characters.
+        private static bool FormatContainsDayOfMonthSpecifier(ReadOnlySpan<char> format)
+        {
+            bool inQuote = false;
+            for (int i = 0; i < format.Length; i++)
+            {
+                char ch = format[i];
+                // Skip the next character if it's escaped
+                if (ch == '\\' || ch == '%')
+                {
+                    i++;
+                    continue;
+                }
+                // Toggle quote state
+                if (ch == '\'' || ch == '"')
+                {
+                    inQuote = !inQuote;
+                    continue;
+                }
+                // Only check for 'd' when not in quotes
+                if (!inQuote && ch == 'd')
+                {
+                    // Make sure it's a day-of-month specifier (d or dd)
+                    // and not a day-of-week specifier (ddd or dddd)
+                    int repeatCount = 1;
+                    while (i + 1 < format.Length && format[i + 1] == 'd')
+                    {
+                        repeatCount++;
+                        i++;
+                    }
+                    // Only day-of-month specifiers (d or dd) trigger genitive case
+                    if (repeatCount <= 2)
+                    {
+                        return true;
+                    }
+                }
+            }
+            return false;
+        }
+
         /*=================================MatchAbbreviatedMonthName==================================
         **Action: Parse the abbreviated month name from string starting at str.Index.
         **Returns: A value from 1 to 12 for the first month to the twelfth month.
@@ -3297,7 +3338,7 @@ private static bool ParseTimeZoneOffset(ref __DTString str, int len, scoped ref
         **Exceptions: FormatException if an abbreviated month name can not be found.
         ==============================================================================*/
 
-        private static bool MatchAbbreviatedMonthName(ref __DTString str, DateTimeFormatInfo dtfi, scoped ref int result)
+        private static bool MatchAbbreviatedMonthName(ref __DTString str, DateTimeFormatInfo dtfi, scoped ref int result, ReadOnlySpan<char> format)
         {
             int maxMatchStrLen = 0;
             result = -1;
@@ -3357,12 +3398,12 @@ private static bool MatchAbbreviatedMonthName(ref __DTString str, DateTimeFormat
                     }
                 }
 
-                // Search genitive form.
-                if ((dtfi.FormatFlags & DateTimeFormatFlags.UseGenitiveMonth) != 0)
+                // Search genitive form only if the format contains a day-of-month specifier
+                bool useDaySpecifier = FormatContainsDayOfMonthSpecifier(format);
+                if ((dtfi.FormatFlags & DateTimeFormatFlags.UseGenitiveMonth) != 0 && useDaySpecifier)
                 {
                     int tempResult = str.MatchLongestWords(dtfi.InternalGetGenitiveMonthNames(abbreviated: true), ref maxMatchStrLen);
-
-                    // We found a longer match in the genitive month name.  Use this as the result.
+                    // We found a longer match in the genitive month name. Use this as the result.
                     // tempResult + 1 should be the month value.
                     if (tempResult >= 0)
                     {
@@ -4045,7 +4086,7 @@ private static bool ParseByFormat(
                     {
                         if (tokenLen == 3)
                         {
-                            if (!MatchAbbreviatedMonthName(ref str, dtfi, ref tempMonth))
+                            if (!MatchAbbreviatedMonthName(ref str, dtfi, ref tempMonth, format.Value))
                             {
                                 result.SetBadDateTimeFailure();
                                 return false;