Fix the ICU time format conversion logic

src/libraries/System.Private.CoreLib/src/System/Globalization/CultureData.Icu.cs

@@ -364,32 +364,59 @@ private static string ConvertIcuTimeFormatString(ReadOnlySpan<char> icuFormatStr
                 char current = icuFormatString[i];
                 switch (current)
                 {
+                    case '\\':
+                        // The ICU format does not use backslashes to escape while the .NET format does
+                        result[resultPos++] = '\\';
+                        result[resultPos++] = '\\';
+                        break;
                     case '\'':
-                        result[resultPos++] = icuFormatString[i++];
-                        while (i < icuFormatString.Length)
+                        static bool HandleQuoteLiteral(ReadOnlySpan<char> icuFormatString, ref int i, Span<char> result, ref int resultPos)
+                        {
+                            if (i + 1 < icuFormatString.Length && icuFormatString[i + 1] == '\'')
+                            {
+                                result[resultPos++] = '\\';
+                                result[resultPos++] = '\'';
+                                i++;
+                                return true;
+                            }
+                            result[resultPos++] = '\'';
+                            return false;
+                        }
+
+                        if (HandleQuoteLiteral(icuFormatString, ref i, result, ref resultPos))
+                        {
+                            break;
+                        }
+                        i++;
+                        for (; i < icuFormatString.Length; i++)
                         {
                             current = icuFormatString[i];
-                            result[resultPos++] = current;
                             if (current == '\'')
                             {
+                                if (HandleQuoteLiteral(icuFormatString, ref i, result, ref resultPos))
+                                {
+                                    continue;
+                                }
                                 break;
                             }
-                            i++;
+                            if (current == '\\')
+                            {
+                                // The same backslash escaping mentioned above
+                                result[resultPos++] = '\\';
+                            }
+                            result[resultPos++] = current;
                         }
                         break;
 
-                    case ':':
-                    case '.':
                     case 'H':
                     case 'h':
                     case 'm':
                     case 's':
-                    case ' ':
-                    case '\u00A0': // no-break space
-                    case '\u202F': // narrow no-break space
                         result[resultPos++] = current;
                         break;
                     case 'a': // AM/PM
+                    case 'b': // am, pm, noon, midnight
+                    case 'B': // flexible day periods
                         if (!amPmAdded)
                         {
                             amPmAdded = true;
@@ -398,6 +425,13 @@ private static string ConvertIcuTimeFormatString(ReadOnlySpan<char> icuFormatStr
                         }
                         break;
 
+                    default:
+                        // Characters that are not ASCII letters are literal texts
+                        if (!char.IsAsciiLetter(current))
+                        {
+                            result[resultPos++] = current;
+                        }
+                        break;
                 }
             }
 

src/libraries/System.Runtime/tests/System.Globalization.Tests/DateTimeFormatInfo/DateTimeFormatInfoData.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Collections.Generic;
 using System.Runtime.InteropServices;
 using Xunit;
 
@@ -58,5 +59,19 @@ public static Exception GetCultureNotSupportedException(CultureInfo cultureInfo)
                 cultureInfo.Name,
                 cultureInfo.Calendar.GetType().Name));
         }
+
+        // These cultures have bad ICU time patterns below the corresponding versions
+        // They are excluded from the VerifyTimePatterns tests
+        public static readonly Dictionary<string, Version> _badIcuTimePatterns = new Dictionary<string, Version>()
+        {
+            { "mi", new Version(65, 0) },
+            { "mi-NZ", new Version(65, 0) },
+        };
+        public static bool HasBadIcuTimePatterns(CultureInfo culture)
+        {
+            return PlatformDetection.IsIcuGlobalizationAndNotHybridOnBrowser
+                && _badIcuTimePatterns.TryGetValue(culture.Name, out var version)
+                && PlatformDetection.ICUVersion < version;
+        }
     }
 }

src/libraries/System.Runtime/tests/System.Globalization.Tests/DateTimeFormatInfo/DateTimeFormatInfoLongTimePattern.cs

@@ -284,6 +284,41 @@ public void LongTimePattern_CheckReadingTimeFormatWithSingleQuotes_ICU()
             }
         }
 
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotHybridGlobalizationOnBrowser))]
+        public void LongTimePattern_VerifyTimePatterns()
+        {
+            Assert.All(CultureInfo.GetCultures(CultureTypes.AllCultures), culture => {
+                if (DateTimeFormatInfoData.HasBadIcuTimePatterns(culture))
+                {
+                    return;
+                }
+                var pattern = culture.DateTimeFormat.LongTimePattern;
+                bool use24Hour = false;
+                bool use12Hour = false;
+                bool useAMPM = false;
+                for (var i = 0; i < pattern.Length; i++)
+                {
+                    switch (pattern[i])
+                    {
+                        case 'H': use24Hour = true; break;
+                        case 'h': use12Hour = true; break;
+                        case 't': useAMPM = true; break;
+                        case '\\': i++; break;
+                        case '\'':
+                            i++;
+                            for (; i < pattern.Length; i++)
+                            {
+                                var c = pattern[i];
+                                if (c == '\'') break;
+                                if (c == '\\') i++;
+                            }
+                            break;
+                    }
+                }
+                Assert.True((use24Hour || useAMPM) && (use12Hour ^ use24Hour), $"Bad long time pattern for culture {culture.Name}: '{pattern}'");
+            });
+        }
+
         [Fact]
         public void LongTimePattern_CheckTimeFormatWithSpaces()
         {

src/libraries/System.Runtime/tests/System.Globalization.Tests/DateTimeFormatInfo/DateTimeFormatInfoShortTimePattern.cs

@@ -255,6 +255,41 @@ public void ShortTimePattern_SetReadOnly_ThrowsInvalidOperationException()
             Assert.Throws<InvalidOperationException>(() => DateTimeFormatInfo.InvariantInfo.ShortTimePattern = "HH:mm");
         }
 
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotHybridGlobalizationOnBrowser))]
+        public void ShortTimePattern_VerifyTimePatterns()
+        {
+            Assert.All(CultureInfo.GetCultures(CultureTypes.AllCultures), culture => {
+                if (DateTimeFormatInfoData.HasBadIcuTimePatterns(culture))
+                {
+                    return;
+                }
+                var pattern = culture.DateTimeFormat.ShortTimePattern;
+                bool use24Hour = false;
+                bool use12Hour = false;
+                bool useAMPM = false;
+                for (var i = 0; i < pattern.Length; i++)
+                {
+                    switch (pattern[i])
+                    {
+                        case 'H': use24Hour = true; break;
+                        case 'h': use12Hour = true; break;
+                        case 't': useAMPM = true; break;
+                        case '\\': i++; break;
+                        case '\'':
+                            i++;
+                            for (; i < pattern.Length; i++)
+                            {
+                                var c = pattern[i];
+                                if (c == '\'') break;
+                                if (c == '\\') i++;
+                            }
+                            break;
+                    }
+                }
+                Assert.True((use24Hour || useAMPM) && (use12Hour ^ use24Hour), $"Bad short time pattern for culture {culture.Name}: '{pattern}'");
+            });
+        }
+
         [Fact]
         public void ShortTimePattern_CheckTimeFormatWithSpaces()
         {