Fix #471 (#473)

[JaEra] Calc: subtracting 1-year from date during Reiwa 1 yields unexpected results.

src/CalcViewModel/Common/DateCalculator.cpp

@@ -20,13 +20,27 @@ DateCalculationEngine::DateCalculationEngine(_In_ String^ calendarIdentifier)
 // Returns: True if function succeeds to calculate the date else returns False
 bool DateCalculationEngine::AddDuration(_In_ DateTime startDate, _In_ const DateDifference& duration, _Out_ DateTime *endDate)
 {
+    auto currentCalendarSystem = m_calendar->GetCalendarSystem();
+
     try
     {
         m_calendar->SetDateTime(startDate);
 
         if (duration.year != 0)
         {
+            // The Japanese Era system can have multiple year partitions within the same year.
+            // For example, April 30, 2019 is denoted April 30, Heisei 31; May 1, 2019 is denoted as May 1, Reiwa 1.
+            // The Calendar treats Heisei 31 and Reiwa 1 as separate years, which results in some unexpected behaviors where subtracting a year from Reiwa 1 results in a date in Heisei 31.
+            // To provide the expected result across era boundaries, we first convert the Japanese era system to a Gregorian system, do date math, and then convert back to the Japanese era system.
+            // This works because the Japanese era system maintains the same year/month boundaries and durations as the Gregorian system and is only different in display value.
+            if (currentCalendarSystem == CalendarIdentifiers::Japanese)
+            {
+                m_calendar->ChangeCalendarSystem(CalendarIdentifiers::Gregorian);
+            }
+
             m_calendar->AddYears(duration.year);
+
+            m_calendar->ChangeCalendarSystem(currentCalendarSystem);
         }
         if (duration.month != 0)
         {
@@ -41,6 +55,9 @@ bool DateCalculationEngine::AddDuration(_In_ DateTime startDate, _In_ const Date
     }
     catch (Platform::InvalidArgumentException^ ex)
     {
+        // ensure that we revert to the correct calendar system
+        m_calendar->ChangeCalendarSystem(currentCalendarSystem);
+
         // Do nothing
         return false;
     }
@@ -52,6 +69,8 @@ bool DateCalculationEngine::AddDuration(_In_ DateTime startDate, _In_ const Date
 // Returns: True if function succeeds to calculate the date else returns False
 bool DateCalculationEngine::SubtractDuration(_In_ DateTime startDate, _In_ const DateDifference& duration, _Out_ DateTime *endDate)
 {
+    auto currentCalendarSystem = m_calendar->GetCalendarSystem();
+
     // For Subtract the Algorithm is different than Add. Here the smaller units are subtracted first
     // and then the larger units.
     try
@@ -68,13 +87,28 @@ bool DateCalculationEngine::SubtractDuration(_In_ DateTime startDate, _In_ const
         }
         if (duration.year != 0)
         {
+            // The Japanese Era system can have multiple year partitions within the same year.
+            // For example, April 30, 2019 is denoted April 30, Heisei 31; May 1, 2019 is denoted as May 1, Reiwa 1.
+            // The Calendar treats Heisei 31 and Reiwa 1 as separate years, which results in some unexpected behaviors where subtracting a year from Reiwa 1 results in a date in Heisei 31.
+            // To provide the expected result across era boundaries, we first convert the Japanese era system to a Gregorian system, do date math, and then convert back to the Japanese era system.
+            // This works because the Japanese era system maintains the same year/month boundaries and durations as the Gregorian system and is only different in display value.
+            if (currentCalendarSystem == CalendarIdentifiers::Japanese)
+            {
+                m_calendar->ChangeCalendarSystem(CalendarIdentifiers::Gregorian);
+            }
+
             m_calendar->AddYears(-duration.year);
+
+            m_calendar->ChangeCalendarSystem(currentCalendarSystem);
         }
 
         *endDate = m_calendar->GetDateTime();
     }
     catch (Platform::InvalidArgumentException^ ex)
     {
+        // ensure that we revert to the correct calendar system
+        m_calendar->ChangeCalendarSystem(currentCalendarSystem);
+
         // Do nothing
         return false;
     }

src/CalculatorUnitTests/DateCalculatorUnitTests.cpp

@@ -664,5 +664,57 @@ namespace DateCalculationUnitTests
                 VERIFY_IS_TRUE(actualValue.find(expectedValue) != wstring::npos, message.c_str());
             }
         }
+
+        TEST_METHOD(JaEraTransitionAddition)
+        {
+            auto viewModel = make_unique<DateCalculationEngine>(CalendarIdentifiers::Japanese);
+            auto cal = ref new Calendar();
+
+            // Showa period ended in Jan 1989.
+            cal->Year = 1989;
+            cal->Month = 1;
+            cal->Day = 1;
+            auto startTime = cal->GetDateTime();
+
+            cal->Year = 1990;
+            cal->Month = 1;
+            cal->Day = 1;
+
+            // Expect that adding a year across boundaries adds the equivalent in the Gregorian calendar.
+            auto expectedResult = cal->GetDateTime();
+            DateDifference duration;
+            duration.year = 1;
+
+            DateTime actualResult;
+            viewModel->AddDuration(startTime, duration, &actualResult);
+
+            VERIFY_ARE_EQUAL(expectedResult.UniversalTime, actualResult.UniversalTime);
+        }
+
+        TEST_METHOD(JaEraTransitionSubtraction)
+        {
+            auto viewModel = make_unique<DateCalculationEngine>(CalendarIdentifiers::Japanese);
+            auto cal = ref new Calendar();
+
+            // Showa period ended in Jan 1989.
+            cal->Year = 1990;
+            cal->Month = 1;
+            cal->Day = 1;
+            auto startTime = cal->GetDateTime();
+
+            cal->Year = 1989;
+            cal->Month = 1;
+            cal->Day = 1;
+
+            // Expect that adding a year across boundaries adds the equivalent in the Gregorian calendar.
+            auto expectedResult = cal->GetDateTime();
+            DateDifference duration;
+            duration.year = 1;
+
+            DateTime actualResult;
+            viewModel->SubtractDuration(startTime, duration, &actualResult);
+
+            VERIFY_ARE_EQUAL(expectedResult.UniversalTime, actualResult.UniversalTime);
+        }
     };
 }