ICU-22633 Fix overflow cause by large AM PM value

Fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=66771

icu4c/source/i18n/calendar.cpp

@@ -3279,7 +3279,9 @@ double Calendar::computeMillisInDay() {
             // Don't normalize here; let overflow bump into the next period.
             // This is consistent with how we handle other fields.
             millisInDay += internalGet(UCAL_HOUR);
-            millisInDay += 12 * internalGet(UCAL_AM_PM); // Default works for unset AM_PM
+            // Treat even number as AM and odd nubmber as PM to align with the
+            // logic in roll()
+            millisInDay += (internalGet(UCAL_AM_PM) % 2 == 0) ? 0 : 12;
         }
     }
 

icu4c/source/test/intltest/caltest.cpp

@@ -195,6 +195,7 @@ void CalendarTest::runIndexedTest( int32_t index, UBool exec, const char* &name,
     TESTCASE_AUTO(Test22633IndianOverflow);
     TESTCASE_AUTO(Test22633IslamicUmalquraOverflow);
     TESTCASE_AUTO(Test22633PersianOverflow);
+    TESTCASE_AUTO(Test22633AMPMOverflow);
 
     TESTCASE_AUTO(TestAddOverflow);
 
@@ -5677,6 +5678,23 @@ void CalendarTest::Test22633PersianOverflow() {
         UCAL_YEAR, status);
     assertFalse("Should not return success", U_SUCCESS(status));
 }
+void CalendarTest::Test22633AMPMOverflow() {
+    UErrorCode status = U_ZERO_ERROR;
+    LocalPointer<Calendar> cal(Calendar::createInstance(Locale("en"), status), status);
+    U_ASSERT(U_SUCCESS(status));
+    cal->setTimeZone(*TimeZone::getGMT());
+    cal->clear();
+    // Test to set a value > limit should not cause internal overflow.
+    cal->set(UCAL_AM_PM, 370633137);
+    assertEquals("set large odd value for UCAL_AM_PM should be treated as PM",
+                 12.0 * 60.0 * 60.0 *1000.0, cal->getTime(status));
+    assertTrue("Should return success", U_SUCCESS(status));
+
+    cal->set(UCAL_AM_PM, 370633138);
+    assertEquals("set large even value for UCAL_AM_PM should be treated as AM",
+                 0.0, cal->getTime(status));
+    assertTrue("Should return success", U_SUCCESS(status));
+}
 
 void CalendarTest::TestChineseCalendarComputeMonthStart() {  // ICU-22639
     UErrorCode status = U_ZERO_ERROR;

icu4c/source/test/intltest/caltest.h

@@ -338,6 +338,7 @@ class CalendarTest: public CalendarTimeZoneTest {
     void Test22633IndianOverflow();
     void Test22633IslamicUmalquraOverflow();
     void Test22633PersianOverflow();
+    void Test22633AMPMOverflow();
 
     void verifyFirstDayOfWeek(const char* locale, UCalendarDaysOfWeek expected);
     void TestFirstDayOfWeek();