Imported Upstream version 6.0.0.172

Former-commit-id: f3cc9b82f3e5bd8f0fd3ebc098f789556b44e9cd

external/corefx/src/System.Globalization/tests/DateTimeFormatInfo/DateTimeFormatInfoTests.cs

@@ -161,5 +161,28 @@ namespace System.Globalization.Tests
                 dt = ci.DateTimeFormat.Calendar.AddMonths(dt, 1);
             }
         }
+
+        [Fact]
+        public void TestFirstYearOfJapaneseEra()
+        {
+            DateTimeFormatInfo jpnFormat = new CultureInfo("ja-JP").DateTimeFormat;
+            jpnFormat.Calendar = new JapaneseCalendar();
+
+            string pattern = "gg yyyy'\u5E74' MM'\u6708' dd'\u65E5'"; // "gg yyyy'年' MM'月' dd'日'"
+            DateTime dt = new DateTime(1989, 01, 08); // Start of Heisei Era
+
+            string formattedDateWithGannen = "\u5E73\u6210 \u5143\u5E74 01\u6708 08\u65E5"; // 平成 元年 01月 08日
+            string formattedDate = dt.ToString(pattern, jpnFormat);
+
+            Assert.True(DateTime.TryParseExact(formattedDate, pattern, jpnFormat, DateTimeStyles.None, out DateTime parsedDate));
+            Assert.Equal(dt, parsedDate);
+
+            // If the formatting with Gan-nen is supported, then parsing should succeed. otherwise parsing should fail.
+            Assert.True(formattedDate.IndexOf("\u5143" /* 元 */, StringComparison.Ordinal) >= 0 ==
+                        DateTime.TryParseExact(formattedDateWithGannen, pattern, jpnFormat, DateTimeStyles.None, out parsedDate),
+                        $"Parsing '{formattedDateWithGannen}' result should match if '{formattedDate}' has Gan-nen symbol"
+                        );
+        }
+
     }
 }