Fix DateTime fraction specifier parsing

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

@@ -4457,16 +4457,14 @@ private static bool ParseByFormat(
                 case '.':
                     if (!str.Match(ch))
                     {
-                        if (format.GetNext())
+                        // If we encounter the pattern ".F", and the dot is not present, it is an optional
+                        // second fraction and we can skip this format.
+                        if (format.Match('F'))
                         {
-                            // If we encounter the pattern ".F", and the dot is not present, it is an optional
-                            // second fraction and we can skip this format.
-                            if (format.Match('F'))
-                            {
-                                format.GetRepeatCount();
-                                break;
-                            }
+                            format.GetRepeatCount();
+                            break;
                         }
+
                         result.SetBadDateTimeFailure();
                         return false;
                     }

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/DateTimeTests.cs

@@ -2763,8 +2763,8 @@ public void TestTimeSynchronizationWithTheSystem()
             DateTime dt = DateTime.UtcNow;
             GetSystemTime(out st1);
 
-            DateTime systemDateTimeNow1  = new DateTime(st.wYear, st.wMonth, st.wDay, st.wHour, st.wMinute, st.wSecond, st.wMillisecond, DateTimeKind.Utc);
-            DateTime systemDateTimeNow2  = new DateTime(st1.wYear, st1.wMonth, st1.wDay, st1.wHour, st1.wMinute, st1.wSecond, st1.wMillisecond, DateTimeKind.Utc);
+            DateTime systemDateTimeNow1 = new DateTime(st.wYear, st.wMonth, st.wDay, st.wHour, st.wMinute, st.wSecond, st.wMillisecond, DateTimeKind.Utc);
+            DateTime systemDateTimeNow2 = new DateTime(st1.wYear, st1.wMonth, st1.wDay, st1.wHour, st1.wMinute, st1.wSecond, st1.wMillisecond, DateTimeKind.Utc);
 
             // Usually GetSystemTime and DateTime.UtcNow calls doesn't take one second to execute, if this is not the case then
             // the thread was sleeping for awhile and we cannot test reliably on that case.
@@ -2777,6 +2777,44 @@ public void TestTimeSynchronizationWithTheSystem()
             }
         }
 
+        [Fact]
+        public void TestFractionSpecifier()
+        {
+            DateTime date = new DateTime(2008, 8, 29, 7, 27, 15, 18);
+            CultureInfo ci = CultureInfo.InvariantCulture;
+
+            // Test using .F format https://learn.microsoft.com/dotnet/standard/base-types/custom-date-and-time-format-strings#F_Specifier
+            string pattern = "yyyy-MM-ddThh:mm:ss.F";
+            string formatted = date.ToString(pattern, ci);
+            Assert.Equal("2008-08-29T07:27:15", formatted);
+            Assert.True(DateTime.TryParseExact(formatted, pattern, ci, DateTimeStyles.None, out DateTime parsedDate));
+            Assert.Equal(date, parsedDate.AddTicks((long)(TimeSpan.TicksPerMillisecond * 18))); // 18 milliseconds fraction difference
+            pattern = "yyyy-MM-ddThh:mm.F:ss";
+            formatted = date.ToString(pattern, ci);
+            Assert.True(DateTime.TryParseExact(formatted, "yyyy-MM-ddThh:mm.F:ss", ci, DateTimeStyles.None, out parsedDate));
+            Assert.Equal(date, parsedDate.AddTicks((long)(TimeSpan.TicksPerMillisecond * 18))); // 18 milliseconds fraction difference
+
+            pattern = "yyyy-MM-ddThh:mm:ss.FF";
+            formatted = date.ToString(pattern, ci);
+            Assert.Equal("2008-08-29T07:27:15.01", formatted);
+            Assert.True(DateTime.TryParseExact(formatted, pattern, ci, DateTimeStyles.None, out parsedDate), $"Failed to parse '{formatted}' using the pattern '{pattern}'.");
+            Assert.Equal(date, parsedDate.AddTicks((long)(TimeSpan.TicksPerMillisecond * 8))); // 8 milliseconds fraction difference
+            pattern = "yyyy-MM-ddThh.FF:mm:ss";
+            formatted = date.ToString(pattern, ci);
+            Assert.True(DateTime.TryParseExact(formatted, pattern, ci, DateTimeStyles.None, out parsedDate), $"Failed to parse '{formatted}' using the pattern '{pattern}'.");
+            Assert.Equal(date, parsedDate.AddTicks((long)(TimeSpan.TicksPerMillisecond * 8))); // 8 milliseconds fraction difference
+
+            pattern = "yyyy-MM-ddThh:mm:ss.FFF";
+            formatted = date.ToString(pattern, ci);
+            Assert.Equal("2008-08-29T07:27:15.018", formatted);
+            Assert.True(DateTime.TryParseExact(formatted, pattern, ci, DateTimeStyles.None, out parsedDate), $"Failed to parse '{formatted}' using the pattern '{pattern}'.");
+            Assert.Equal(date, parsedDate);
+            pattern = "yyyy-MM-ddThh.FFF:mm:ss";
+            formatted = date.ToString(pattern, ci);
+            Assert.True(DateTime.TryParseExact(formatted, pattern, ci, DateTimeStyles.None, out parsedDate), $"Failed to parse '{formatted}' using the pattern '{pattern}'.");
+            Assert.Equal(date, parsedDate);
+        }
+
         [DllImport("Kernel32.dll")]
         internal static extern void GetSystemTime(out SYSTEMTIME lpSystemTime);