1 files changed, 621 insertions, 0 deletions
diff --git a/intl/components/gtest/TestDateTimeFormat.cpp b/intl/components/gtest/TestDateTimeFormat.cpp
new file mode 100644
index 0000000000..057e26ea13
--- /dev/null
+++ b/intl/components/gtest/TestDateTimeFormat.cpp
@@ -0,0 +1,621 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#include "gtest/gtest.h"
+
+#include "mozilla/intl/Calendar.h"
+#include "mozilla/intl/DateTimeFormat.h"
+#include "mozilla/intl/DateTimePart.h"
+#include "mozilla/intl/DateTimePatternGenerator.h"
+#include "mozilla/Span.h"
+#include "TestBuffer.h"
+
+#include <string_view>
+
+namespace mozilla::intl {
+
+// Firefox 1.0 release date.
+const double DATE = 1032800850000.0;
+
+static UniquePtr<DateTimeFormat> testStyle(
+    const char* aLocale, DateTimeFormat::StyleBag& aStyleBag) {
+  // Always specify a time zone in the tests, otherwise it will use the system
+  // time zone which can vary between test runs.
+  auto timeZone = Some(MakeStringSpan(u"GMT+3"));
+  auto gen = DateTimePatternGenerator::TryCreate("en").unwrap();
+  return DateTimeFormat::TryCreateFromStyle(MakeStringSpan(aLocale), aStyleBag,
+                                            gen.get(), timeZone)
+      .unwrap();
+}
+
+TEST(IntlDateTimeFormat, Style_enUS_utf8)
+{
+  DateTimeFormat::StyleBag style;
+  style.date = Some(DateTimeFormat::Style::Medium);
+  style.time = Some(DateTimeFormat::Style::Medium);
+
+  auto dtFormat = testStyle("en-US", style);
+  TestBuffer<char> buffer;
+  dtFormat->TryFormat(DATE, buffer).unwrap();
+
+  ASSERT_TRUE(buffer.verboseMatches("Sep 23, 2002, 8:07:30 PM"));
+}
+
+TEST(IntlDateTimeFormat, Style_enUS_utf16)
+{
+  DateTimeFormat::StyleBag style;
+  style.date = Some(DateTimeFormat::Style::Medium);
+  style.time = Some(DateTimeFormat::Style::Medium);
+
+  auto dtFormat = testStyle("en-US", style);
+  TestBuffer<char16_t> buffer;
+  dtFormat->TryFormat(DATE, buffer).unwrap();
+
+  ASSERT_TRUE(buffer.verboseMatches(u"Sep 23, 2002, 8:07:30 PM"));
+}
+
+TEST(IntlDateTimeFormat, Style_ar_utf8)
+{
+  DateTimeFormat::StyleBag style;
+  style.time = Some(DateTimeFormat::Style::Medium);
+
+  auto dtFormat = testStyle("ar", style);
+  TestBuffer<char> buffer;
+  dtFormat->TryFormat(DATE, buffer).unwrap();
+
+  ASSERT_TRUE(buffer.verboseMatches("٨:٠٧:٣٠ م"));
+}
+
+TEST(IntlDateTimeFormat, Style_ar_utf16)
+{
+  DateTimeFormat::StyleBag style;
+  style.time = Some(DateTimeFormat::Style::Medium);
+
+  auto dtFormat = testStyle("ar", style);
+  TestBuffer<char16_t> buffer;
+  dtFormat->TryFormat(DATE, buffer).unwrap();
+
+  ASSERT_TRUE(buffer.verboseMatches(u"٨:٠٧:٣٠ م"));
+}
+
+TEST(IntlDateTimeFormat, Style_enUS_fallback_to_default_styles)
+{
+  DateTimeFormat::StyleBag style;
+
+  auto dtFormat = testStyle("en-US", style);
+  TestBuffer<char> buffer;
+  dtFormat->TryFormat(DATE, buffer).unwrap();
+
+  ASSERT_TRUE(buffer.verboseMatches("Sep 23, 2002, 8:07:30 PM"));
+}
+
+TEST(IntlDateTimeFormat, Time_zone_IANA_identifier)
+{
+  auto gen = DateTimePatternGenerator::TryCreate("en").unwrap();
+
+  DateTimeFormat::StyleBag style;
+  style.date = Some(DateTimeFormat::Style::Medium);
+  style.time = Some(DateTimeFormat::Style::Medium);
+
+  auto dtFormat = DateTimeFormat::TryCreateFromStyle(
+                      MakeStringSpan("en-US"), style, gen.get(),
+                      Some(MakeStringSpan(u"America/Chicago")))
+                      .unwrap();
+  TestBuffer<char> buffer;
+  dtFormat->TryFormat(DATE, buffer).unwrap();
+  ASSERT_TRUE(buffer.verboseMatches("Sep 23, 2002, 12:07:30 PM"));
+}
+
+TEST(IntlDateTimeFormat, GetAllowedHourCycles)
+{
+  auto allowed_en_US = DateTimeFormat::GetAllowedHourCycles(
+                           MakeStringSpan("en"), Some(MakeStringSpan("US")))
+                           .unwrap();
+
+  ASSERT_TRUE(allowed_en_US.length() == 2);
+  ASSERT_EQ(allowed_en_US[0], DateTimeFormat::HourCycle::H12);
+  ASSERT_EQ(allowed_en_US[1], DateTimeFormat::HourCycle::H23);
+
+  auto allowed_de =
+      DateTimeFormat::GetAllowedHourCycles(MakeStringSpan("de"), Nothing())
+          .unwrap();
+
+  ASSERT_TRUE(allowed_de.length() == 2);
+  ASSERT_EQ(allowed_de[0], DateTimeFormat::HourCycle::H23);
+  ASSERT_EQ(allowed_de[1], DateTimeFormat::HourCycle::H12);
+}
+
+TEST(IntlDateTimePatternGenerator, GetBestPattern)
+{
+  auto gen = DateTimePatternGenerator::TryCreate("en").unwrap();
+  TestBuffer<char16_t> buffer;
+
+  gen->GetBestPattern(MakeStringSpan(u"yMd"), buffer).unwrap();
+  ASSERT_TRUE(buffer.verboseMatches(u"M/d/y"));
+}
+
+TEST(IntlDateTimePatternGenerator, GetSkeleton)
+{
+  auto gen = DateTimePatternGenerator::TryCreate("en").unwrap();
+  TestBuffer<char16_t> buffer;
+
+  DateTimePatternGenerator::GetSkeleton(MakeStringSpan(u"M/d/y"), buffer)
+      .unwrap();
+  ASSERT_TRUE(buffer.verboseMatches(u"yMd"));
+}
+
+TEST(IntlDateTimePatternGenerator, GetPlaceholderPattern)
+{
+  auto gen = DateTimePatternGenerator::TryCreate("en").unwrap();
+  auto span = gen->GetPlaceholderPattern();
+  // The default date-time pattern for 'en' locale is u"{1}, {0}".
+  ASSERT_EQ(span, MakeStringSpan(u"{1}, {0}"));
+}
+
+// A utility function to help test the DateTimeFormat::ComponentsBag.
+[[nodiscard]] bool FormatComponents(
+    TestBuffer<char16_t>& aBuffer, DateTimeFormat::ComponentsBag& aComponents,
+    Span<const char> aLocale = MakeStringSpan("en-US")) {
+  UniquePtr<DateTimePatternGenerator> gen = nullptr;
+  auto dateTimePatternGenerator =
+      DateTimePatternGenerator::TryCreate(aLocale.data()).unwrap();
+
+  auto dtFormat = DateTimeFormat::TryCreateFromComponents(
+      aLocale, aComponents, dateTimePatternGenerator.get(),
+      Some(MakeStringSpan(u"GMT+3")));
+  if (dtFormat.isErr()) {
+    fprintf(stderr, "Could not create a DateTimeFormat\n");
+    return false;
+  }
+
+  auto result = dtFormat.unwrap()->TryFormat(DATE, aBuffer);
+  if (result.isErr()) {
+    fprintf(stderr, "Could not format a DateTimeFormat\n");
+    return false;
+  }
+
+  return true;
+}
+
+TEST(IntlDateTimeFormat, Components)
+{
+  DateTimeFormat::ComponentsBag components{};
+
+  components.year = Some(DateTimeFormat::Numeric::Numeric);
+  components.month = Some(DateTimeFormat::Month::Numeric);
+  components.day = Some(DateTimeFormat::Numeric::Numeric);
+
+  components.hour = Some(DateTimeFormat::Numeric::Numeric);
+  components.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.second = Some(DateTimeFormat::Numeric::TwoDigit);
+
+  TestBuffer<char16_t> buffer;
+  ASSERT_TRUE(FormatComponents(buffer, components));
+  ASSERT_TRUE(buffer.verboseMatches(u"9/23/2002, 8:07:30 PM"));
+}
+
+TEST(IntlDateTimeFormat, Components_es_ES)
+{
+  DateTimeFormat::ComponentsBag components{};
+
+  components.year = Some(DateTimeFormat::Numeric::Numeric);
+  components.month = Some(DateTimeFormat::Month::Numeric);
+  components.day = Some(DateTimeFormat::Numeric::Numeric);
+
+  components.hour = Some(DateTimeFormat::Numeric::Numeric);
+  components.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.second = Some(DateTimeFormat::Numeric::TwoDigit);
+
+  TestBuffer<char16_t> buffer;
+  ASSERT_TRUE(FormatComponents(buffer, components, MakeStringSpan("es-ES")));
+  ASSERT_TRUE(buffer.verboseMatches(u"23/9/2002, 20:07:30"));
+}
+
+TEST(IntlDateTimeFormat, ComponentsAll)
+{
+  // Use most all of the components.
+  DateTimeFormat::ComponentsBag components{};
+
+  components.era = Some(DateTimeFormat::Text::Short);
+
+  components.year = Some(DateTimeFormat::Numeric::Numeric);
+  components.month = Some(DateTimeFormat::Month::Numeric);
+  components.day = Some(DateTimeFormat::Numeric::Numeric);
+
+  components.weekday = Some(DateTimeFormat::Text::Short);
+
+  components.hour = Some(DateTimeFormat::Numeric::Numeric);
+  components.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.second = Some(DateTimeFormat::Numeric::TwoDigit);
+
+  components.timeZoneName = Some(DateTimeFormat::TimeZoneName::Short);
+  components.hourCycle = Some(DateTimeFormat::HourCycle::H24);
+  components.fractionalSecondDigits = Some(3);
+
+  TestBuffer<char16_t> buffer;
+  ASSERT_TRUE(FormatComponents(buffer, components));
+  ASSERT_TRUE(buffer.verboseMatches(u"Mon, 9 23, 2002 AD, 20:07:30.000 GMT+3"));
+}
+
+TEST(IntlDateTimeFormat, ComponentsHour12Default)
+{
+  // Assert the behavior of the default "en-US" 12 hour time with day period.
+  DateTimeFormat::ComponentsBag components{};
+  components.hour = Some(DateTimeFormat::Numeric::Numeric);
+  components.minute = Some(DateTimeFormat::Numeric::Numeric);
+
+  TestBuffer<char16_t> buffer;
+  ASSERT_TRUE(FormatComponents(buffer, components));
+  ASSERT_TRUE(buffer.verboseMatches(u"8:07 PM"));
+}
+
+TEST(IntlDateTimeFormat, ComponentsHour24)
+{
+  // Test the behavior of using 24 hour time to override the default of
+  // hour 12 with a day period.
+  DateTimeFormat::ComponentsBag components{};
+  components.hour = Some(DateTimeFormat::Numeric::Numeric);
+  components.minute = Some(DateTimeFormat::Numeric::Numeric);
+  components.hour12 = Some(false);
+
+  TestBuffer<char16_t> buffer;
+  ASSERT_TRUE(FormatComponents(buffer, components));
+  ASSERT_TRUE(buffer.verboseMatches(u"20:07"));
+}
+
+TEST(IntlDateTimeFormat, ComponentsHour12DayPeriod)
+{
+  // Test the behavior of specifying a specific day period.
+  DateTimeFormat::ComponentsBag components{};
+
+  components.hour = Some(DateTimeFormat::Numeric::Numeric);
+  components.minute = Some(DateTimeFormat::Numeric::Numeric);
+  components.dayPeriod = Some(DateTimeFormat::Text::Long);
+
+  TestBuffer<char16_t> buffer;
+  ASSERT_TRUE(FormatComponents(buffer, components));
+  ASSERT_TRUE(buffer.verboseMatches(u"8:07 in the evening"));
+}
+
+const char* ToString(uint8_t b) { return "uint8_t"; }
+const char* ToString(bool b) { return b ? "true" : "false"; }
+
+template <typename T>
+const char* ToString(Maybe<T> option) {
+  if (option) {
+    if constexpr (std::is_same_v<T, bool> || std::is_same_v<T, uint8_t>) {
+      return ToString(*option);
+    } else {
+      return DateTimeFormat::ToString(*option);
+    }
+  }
+  return "Nothing";
+}
+
+template <typename T>
+[[nodiscard]] bool VerboseEquals(T expected, T actual, const char* msg) {
+  if (expected != actual) {
+    fprintf(stderr, "%s\n  Actual: %s\nExpected: %s\n", msg, ToString(actual),
+            ToString(expected));
+    return false;
+  }
+  return true;
+}
+
+// A testing utility for getting nice errors when ComponentsBags don't match.
+[[nodiscard]] bool VerboseEquals(DateTimeFormat::ComponentsBag& expected,
+                                 DateTimeFormat::ComponentsBag& actual) {
+  // clang-format off
+  return
+      VerboseEquals(expected.era, actual.era, "Components do not match: bag.era") &&
+      VerboseEquals(expected.year, actual.year, "Components do not match: bag.year") &&
+      VerboseEquals(expected.month, actual.month, "Components do not match: bag.month") &&
+      VerboseEquals(expected.day, actual.day, "Components do not match: bag.day") &&
+      VerboseEquals(expected.weekday, actual.weekday, "Components do not match: bag.weekday") &&
+      VerboseEquals(expected.hour, actual.hour, "Components do not match: bag.hour") &&
+      VerboseEquals(expected.minute, actual.minute, "Components do not match: bag.minute") &&
+      VerboseEquals(expected.second, actual.second, "Components do not match: bag.second") &&
+      VerboseEquals(expected.timeZoneName, actual.timeZoneName, "Components do not match: bag.timeZoneName") &&
+      VerboseEquals(expected.hour12, actual.hour12, "Components do not match: bag.hour12") &&
+      VerboseEquals(expected.hourCycle, actual.hourCycle, "Components do not match: bag.hourCycle") &&
+      VerboseEquals(expected.dayPeriod, actual.dayPeriod, "Components do not match: bag.dayPeriod") &&
+      VerboseEquals(expected.fractionalSecondDigits, actual.fractionalSecondDigits, "Components do not match: bag.fractionalSecondDigits");
+  // clang-format on
+}
+
+// A utility function to help test the DateTimeFormat::ComponentsBag.
+[[nodiscard]] bool ResolveComponentsBag(
+    DateTimeFormat::ComponentsBag& aComponentsIn,
+    DateTimeFormat::ComponentsBag* aComponentsOut,
+    Span<const char> aLocale = MakeStringSpan("en-US")) {
+  UniquePtr<DateTimePatternGenerator> gen = nullptr;
+  auto dateTimePatternGenerator =
+      DateTimePatternGenerator::TryCreate("en").unwrap();
+  auto dtFormat = DateTimeFormat::TryCreateFromComponents(
+      aLocale, aComponentsIn, dateTimePatternGenerator.get(),
+      Some(MakeStringSpan(u"GMT+3")));
+  if (dtFormat.isErr()) {
+    fprintf(stderr, "Could not create a DateTimeFormat\n");
+    return false;
+  }
+
+  auto result = dtFormat.unwrap()->ResolveComponents();
+  if (result.isErr()) {
+    fprintf(stderr, "Could not resolve the components\n");
+    return false;
+  }
+
+  *aComponentsOut = result.unwrap();
+  return true;
+}
+
+TEST(IntlDateTimeFormat, ResolvedComponentsDate)
+{
+  DateTimeFormat::ComponentsBag input{};
+  {
+    input.year = Some(DateTimeFormat::Numeric::Numeric);
+    input.month = Some(DateTimeFormat::Month::Numeric);
+    input.day = Some(DateTimeFormat::Numeric::Numeric);
+  }
+
+  DateTimeFormat::ComponentsBag expected = input;
+
+  DateTimeFormat::ComponentsBag resolved{};
+  ASSERT_TRUE(ResolveComponentsBag(input, &resolved));
+  ASSERT_TRUE(VerboseEquals(expected, resolved));
+}
+
+TEST(IntlDateTimeFormat, ResolvedComponentsAll)
+{
+  DateTimeFormat::ComponentsBag input{};
+  {
+    input.era = Some(DateTimeFormat::Text::Short);
+
+    input.year = Some(DateTimeFormat::Numeric::Numeric);
+    input.month = Some(DateTimeFormat::Month::Numeric);
+    input.day = Some(DateTimeFormat::Numeric::Numeric);
+
+    input.weekday = Some(DateTimeFormat::Text::Short);
+
+    input.hour = Some(DateTimeFormat::Numeric::Numeric);
+    input.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+    input.second = Some(DateTimeFormat::Numeric::TwoDigit);
+
+    input.timeZoneName = Some(DateTimeFormat::TimeZoneName::Short);
+    input.hourCycle = Some(DateTimeFormat::HourCycle::H24);
+    input.fractionalSecondDigits = Some(3);
+  }
+
+  DateTimeFormat::ComponentsBag expected = input;
+  {
+    expected.hour = Some(DateTimeFormat::Numeric::TwoDigit);
+    expected.hourCycle = Some(DateTimeFormat::HourCycle::H24);
+    expected.hour12 = Some(false);
+  }
+
+  DateTimeFormat::ComponentsBag resolved{};
+  ASSERT_TRUE(ResolveComponentsBag(input, &resolved));
+  ASSERT_TRUE(VerboseEquals(expected, resolved));
+}
+
+TEST(IntlDateTimeFormat, ResolvedComponentsHourDayPeriod)
+{
+  DateTimeFormat::ComponentsBag input{};
+  {
+    input.hour = Some(DateTimeFormat::Numeric::Numeric);
+    input.minute = Some(DateTimeFormat::Numeric::Numeric);
+  }
+
+  DateTimeFormat::ComponentsBag expected = input;
+  {
+    expected.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+    expected.hourCycle = Some(DateTimeFormat::HourCycle::H12);
+    expected.hour12 = Some(true);
+  }
+
+  DateTimeFormat::ComponentsBag resolved{};
+  ASSERT_TRUE(ResolveComponentsBag(input, &resolved));
+  ASSERT_TRUE(VerboseEquals(expected, resolved));
+}
+
+TEST(IntlDateTimeFormat, ResolvedComponentsHour12)
+{
+  DateTimeFormat::ComponentsBag input{};
+  {
+    input.hour = Some(DateTimeFormat::Numeric::Numeric);
+    input.minute = Some(DateTimeFormat::Numeric::Numeric);
+    input.hour12 = Some(false);
+  }
+
+  DateTimeFormat::ComponentsBag expected = input;
+  {
+    expected.hour = Some(DateTimeFormat::Numeric::TwoDigit);
+    expected.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+    expected.hourCycle = Some(DateTimeFormat::HourCycle::H23);
+    expected.hour12 = Some(false);
+  }
+
+  DateTimeFormat::ComponentsBag resolved{};
+  ASSERT_TRUE(ResolveComponentsBag(input, &resolved));
+  ASSERT_TRUE(VerboseEquals(expected, resolved));
+}
+
+TEST(IntlDateTimeFormat, GetOriginalSkeleton)
+{
+  // Demonstrate that the original skeleton and the resolved skeleton can
+  // differ.
+  DateTimeFormat::ComponentsBag components{};
+  components.month = Some(DateTimeFormat::Month::Narrow);
+  components.day = Some(DateTimeFormat::Numeric::TwoDigit);
+
+  const char* locale = "zh-Hans-CN";
+  auto dateTimePatternGenerator =
+      DateTimePatternGenerator::TryCreate(locale).unwrap();
+
+  auto result = DateTimeFormat::TryCreateFromComponents(
+      MakeStringSpan(locale), components, dateTimePatternGenerator.get(),
+      Some(MakeStringSpan(u"GMT+3")));
+  ASSERT_TRUE(result.isOk());
+  auto dtFormat = result.unwrap();
+
+  TestBuffer<char16_t> originalSkeleton;
+  auto originalSkeletonResult = dtFormat->GetOriginalSkeleton(originalSkeleton);
+  ASSERT_TRUE(originalSkeletonResult.isOk());
+  ASSERT_TRUE(originalSkeleton.verboseMatches(u"MMMMMdd"));
+
+  TestBuffer<char16_t> pattern;
+  auto patternResult = dtFormat->GetPattern(pattern);
+  ASSERT_TRUE(patternResult.isOk());
+  ASSERT_TRUE(pattern.verboseMatches(u"M月dd日"));
+
+  TestBuffer<char16_t> resolvedSkeleton;
+  auto resolvedSkeletonResult = DateTimePatternGenerator::GetSkeleton(
+      Span(pattern.data(), pattern.length()), resolvedSkeleton);
+
+  ASSERT_TRUE(resolvedSkeletonResult.isOk());
+  ASSERT_TRUE(resolvedSkeleton.verboseMatches(u"Mdd"));
+}
+
+TEST(IntlDateTimeFormat, GetAvailableLocales)
+{
+  using namespace std::literals;
+
+  int32_t english = 0;
+  int32_t german = 0;
+  int32_t chinese = 0;
+
+  // Since this list is dependent on ICU, and may change between upgrades, only
+  // test a subset of the available locales.
+  for (const char* locale : DateTimeFormat::GetAvailableLocales()) {
+    if (locale == "en"sv) {
+      english++;
+    } else if (locale == "de"sv) {
+      german++;
+    } else if (locale == "zh"sv) {
+      chinese++;
+    }
+  }
+
+  // Each locale should be found exactly once.
+  ASSERT_EQ(english, 1);
+  ASSERT_EQ(german, 1);
+  ASSERT_EQ(chinese, 1);
+}
+
+TEST(IntlDateTimeFormat, TryFormatToParts)
+{
+  auto dateTimePatternGenerator =
+      DateTimePatternGenerator::TryCreate("en").unwrap();
+
+  DateTimeFormat::ComponentsBag components;
+  components.year = Some(DateTimeFormat::Numeric::Numeric);
+  components.month = Some(DateTimeFormat::Month::TwoDigit);
+  components.day = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.hour = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.hour12 = Some(false);
+
+  UniquePtr<DateTimeFormat> dtFormat =
+      DateTimeFormat::TryCreateFromComponents(
+          MakeStringSpan("en-US"), components, dateTimePatternGenerator.get(),
+          Some(MakeStringSpan(u"GMT")))
+          .unwrap();
+
+  TestBuffer<char16_t> buffer;
+  mozilla::intl::DateTimePartVector parts;
+  auto result = dtFormat->TryFormatToParts(DATE, buffer, parts);
+  ASSERT_TRUE(result.isOk());
+
+  std::u16string_view strView = buffer.get_string_view();
+  ASSERT_EQ(strView, u"09/23/2002, 17:07");
+
+  auto getSubStringView = [strView, &parts](size_t index) {
+    size_t pos = index == 0 ? 0 : parts[index - 1].mEndIndex;
+    size_t count = parts[index].mEndIndex - pos;
+    return strView.substr(pos, count);
+  };
+
+  ASSERT_EQ(parts[0].mType, DateTimePartType::Month);
+  ASSERT_EQ(getSubStringView(0), u"09");
+
+  ASSERT_EQ(parts[1].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubStringView(1), u"/");
+
+  ASSERT_EQ(parts[2].mType, DateTimePartType::Day);
+  ASSERT_EQ(getSubStringView(2), u"23");
+
+  ASSERT_EQ(parts[3].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubStringView(3), u"/");
+
+  ASSERT_EQ(parts[4].mType, DateTimePartType::Year);
+  ASSERT_EQ(getSubStringView(4), u"2002");
+
+  ASSERT_EQ(parts[5].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubStringView(5), u", ");
+
+  ASSERT_EQ(parts[6].mType, DateTimePartType::Hour);
+  ASSERT_EQ(getSubStringView(6), u"17");
+
+  ASSERT_EQ(parts[7].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubStringView(7), u":");
+
+  ASSERT_EQ(parts[8].mType, DateTimePartType::Minute);
+  ASSERT_EQ(getSubStringView(8), u"07");
+
+  ASSERT_EQ(parts.length(), 9u);
+}
+
+TEST(IntlDateTimeFormat, SetStartTimeIfGregorian)
+{
+  DateTimeFormat::StyleBag style{};
+  style.date = Some(DateTimeFormat::Style::Long);
+
+  auto timeZone = Some(MakeStringSpan(u"UTC"));
+
+  // Beginning of ECMAScript time.
+  constexpr double StartOfTime = -8.64e15;
+
+  // Gregorian change date defaults to October 15, 1582 in ICU. Test with a date
+  // before the default change date, in this case January 1, 1582.
+  constexpr double FirstJanuary1582 = -12244089600000.0;
+
+  // One year expressed in milliseconds.
+  constexpr double oneYear = (365 * 24 * 60 * 60) * 1000.0;
+
+  // Test with and without explicit calendar. The start time of the calendar can
+  // only be adjusted for the Gregorian and the ISO-8601 calendar.
+  for (const char* locale : {
+           "en-US",
+           "en-US-u-ca-gregory",
+           "en-US-u-ca-iso8601",
+       }) {
+    auto gen = DateTimePatternGenerator::TryCreate(locale).unwrap();
+
+    auto dtFormat = DateTimeFormat::TryCreateFromStyle(
+                        MakeStringSpan(locale), style, gen.get(), timeZone)
+                        .unwrap();
+
+    TestBuffer<char> buffer;
+
+    // Before the default Gregorian change date, so interpreted in the Julian
+    // calendar, which is December 22, 1581.
+    dtFormat->TryFormat(FirstJanuary1582, buffer).unwrap();
+    ASSERT_TRUE(buffer.verboseMatches("December 22, 1581"));
+
+    // After default Gregorian change date, so January 1, 1583.
+    dtFormat->TryFormat(FirstJanuary1582 + oneYear, buffer).unwrap();
+    ASSERT_TRUE(buffer.verboseMatches("January 1, 1583"));
+
+    // Adjust the start time to use a proleptic Gregorian calendar.
+    dtFormat->SetStartTimeIfGregorian(StartOfTime);
+
+    // Now interpreted in proleptic Gregorian calendar at January 1, 1582.
+    dtFormat->TryFormat(FirstJanuary1582, buffer).unwrap();
+    ASSERT_TRUE(buffer.verboseMatches("January 1, 1582"));
+
+    // Still January 1, 1583.
+    dtFormat->TryFormat(FirstJanuary1582 + oneYear, buffer).unwrap();
+    ASSERT_TRUE(buffer.verboseMatches("January 1, 1583"));
+  }
+}
+}  // namespace mozilla::intl