Clarify MatchAbbreviatedMonthName comment

src/libraries/System.Globalization.Calendars/tests/Misc/MiscCalendars.cs

@@ -42,5 +42,19 @@ public static void JapaneseTest()
             DateTime dTest = jCal.ToDateTime(1, 1, 8, 0, 0, 0, 0, 4);
             Assert.Equal(dTest, new DateTime(1989, 1, 8));
         }
+
+        [Theory]
+        [InlineData(@"Thg 1", 1)]
+        [InlineData(@"Thg 2", 2)]
+        [InlineData(@"Thg 10", 10)]
+        [InlineData(@"Thg 11", 11)]
+        [InlineData(@"Thg 12", 12)]
+        //[InlineData(@"Thg 13", 13)] Needs supported calendar with 13 months
+        public static void VietnameseTest_MatchAbbreviatedMonthName_ContinuesSearchAfterMatch(string monthAbberavition, int expectedMonthNum)
+        {
+            var dtfi = new System.Globalization.CultureInfo("vi-VN").DateTimeFormat;
+            dtfi.AbbreviatedMonthNames = new string[] { "Thg 1", "Thg 2", "Thg 3", "Thg 4", "Thg 5", "Thg 6", "Thg 7", "Thg 8", "Thg 9", "Thg 10", "Thg 11", "Thg 12", "Thg 13" }; // 13 months
+            Assert.Equal(expectedMonthNum, DateTime.ParseExact(monthAbberavition, "MMM", dtfi).Month);
+        }
     }
 }

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

@@ -3352,9 +3352,9 @@ private static bool MatchAbbreviatedMonthName(ref __DTString str, DateTimeFormat
                 {
                     // Scan the month names (note that some calendars has 13 months) and find
                     // the matching month name which has the max string length.
-                    // We need to do this because some cultures (e.g. "cs-CZ") which have
-                    // abbreviated month names with the same prefix.
-                    int monthsInYear = (dtfi.GetMonthName(13).Length == 0 ? 12 : 13);
+                    // We need to do this because some cultures which have abbreviated
+                    // month names with the same prefix (e.g. "vi-VN" culture has those conflicts "Thg1", "Thg10", "Thg11")
+                    int monthsInYear = (dtfi.GetAbbreviatedMonthName(13).Length == 0 ? 12 : 13);
                     for (int i = 1; i <= monthsInYear; i++)
                     {
                         string searchStr = dtfi.GetAbbreviatedMonthName(i);