# Use Gregorian eras for pre-Meiji dates in the Japanese calendar.

diff --git a/intl/icu/source/data/locales/root.txt b/intl/icu/source/data/locales/root.txt
--- a/intl/icu/source/data/locales/root.txt
+++ b/intl/icu/source/data/locales/root.txt
@@ -2956,23 +2956,27 @@ root{
             DateTimePatterns:alias{"/LOCALE/calendar/generic/DateTimePatterns"}
             NoonMarker:alias{"/LOCALE/calendar/gregorian/NoonMarker"}
             NoonMarkerNarrow:alias{"/LOCALE/calendar/gregorian/NoonMarkerNarrow"}
             appendItems:alias{"/LOCALE/calendar/generic/appendItems"}
             availableFormats:alias{"/LOCALE/calendar/generic/availableFormats"}
             dayNames:alias{"/LOCALE/calendar/gregorian/dayNames"}
             eras{
                 abbreviated{
+                    0:alias{"/LOCALE/calendar/gregorian/eras/abbreviated/0"}
+                    1:alias{"/LOCALE/calendar/gregorian/eras/abbreviated/1"}
                     232{"Meiji"}
                     233{"Taishō"}
                     234{"Shōwa"}
                     235{"Heisei"}
                     236{"Reiwa"}
                 }
                 narrow{
+                    0:alias{"/LOCALE/calendar/gregorian/eras/narrow/0"}
+                    1:alias{"/LOCALE/calendar/gregorian/eras/narrow/1"}
                     232{"M"}
                     233{"T"}
                     234{"S"}
                     235{"H"}
                     236{"R"}
                 }
                 wide:alias{"/LOCALE/calendar/japanese/eras/abbreviated"}
             }
diff --git a/intl/icu/source/data/misc/supplementalData.txt b/intl/icu/source/data/misc/supplementalData.txt
--- a/intl/icu/source/data/misc/supplementalData.txt
+++ b/intl/icu/source/data/misc/supplementalData.txt
@@ -234,16 +234,30 @@ supplementalData:table(nofallback){
                         1,
                     }
                 }
             }
             system{"solar"}
         }
         japanese{
             eras{
+                0{
+                    end:intvector{
+                        0,
+                        12,
+                        31,
+                    }
+                }
+                1{
+                    start:intvector{
+                        1,
+                        1,
+                        1,
+                    }
+                }
                 232{
                     start:intvector{
                         1868,
                         10,
                         23,
                     }
                 }
                 233{
diff --git a/intl/icu/source/i18n/dtfmtsym.cpp b/intl/icu/source/i18n/dtfmtsym.cpp
--- a/intl/icu/source/i18n/dtfmtsym.cpp
+++ b/intl/icu/source/i18n/dtfmtsym.cpp
@@ -1735,17 +1735,17 @@ struct CalendarDataSink : public Resourc
             // Ignore '%variant' keys
             if (keyUString.endsWith(kVariantTagUChar, UPRV_LENGTHOF(kVariantTagUChar))) {
                 continue;
             }
 
             // == Handle String elements ==
             if (value.getType() == URES_STRING) {
                 // We are on a leaf, store the map elements into the stringMap
-                if (i == 0) {
+                if (stringMap == nullptr) {
                     // mapRefs will keep ownership of 'stringMap':
                     stringMap = mapRefs.create(false, errorCode);
                     if (stringMap == nullptr) {
                         errorCode = U_MEMORY_ALLOCATION_ERROR;
                         return;
                     }
                     maps.put(path, stringMap, errorCode);
                     if (U_FAILURE(errorCode)) { return; }
@@ -1799,17 +1799,17 @@ struct CalendarDataSink : public Resourc
             if (arrays.get(path) != nullptr || maps.get(path) != nullptr) {
                 // Drop the latest key on the path and continue
                 path.retainBetween(0, pathLength);
                 continue;
             }
 
             AliasType aliasType = processAliasFromValue(path, value, errorCode);
             if (U_FAILURE(errorCode)) { return; }
-            if (aliasType == SAME_CALENDAR) {
+            if (aliasType != NONE) {
                 // Store the alias path and the current path on aliasPathPairs
                 LocalPointer<UnicodeString> aliasRelativePathCopy(aliasRelativePath.clone(), errorCode);
                 aliasPathPairs.adoptElement(aliasRelativePathCopy.orphan(), errorCode);
                 if (U_FAILURE(errorCode)) { return; }
                 LocalPointer<UnicodeString> pathCopy(path.clone(), errorCode);
                 aliasPathPairs.adoptElement(pathCopy.orphan(), errorCode);
                 if (U_FAILURE(errorCode)) { return; }
 
diff --git a/intl/icu/source/i18n/japancal.cpp b/intl/icu/source/i18n/japancal.cpp
--- a/intl/icu/source/i18n/japancal.cpp
+++ b/intl/icu/source/i18n/japancal.cpp
@@ -219,16 +219,21 @@ int32_t JapaneseCalendar::handleGetExten
 
 void JapaneseCalendar::handleComputeFields(int32_t julianDay, UErrorCode& status)
 {
     //Calendar::timeToFields(theTime, quick, status);
     GregorianCalendar::handleComputeFields(julianDay, status);
     int32_t year = internalGet(UCAL_EXTENDED_YEAR); // Gregorian year
     int32_t eraCode = gJapaneseEraRules->getEraCode(year, internalGetMonth(status) + 1, internalGet(UCAL_DAY_OF_MONTH), status);
 
+    // No adjustment necessary for BC and AD eras.
+    if (eraCode == EEras::BC || eraCode == EEras::AD) {
+        return;
+    }
+
     int32_t startYear = gJapaneseEraRules->getStartYear(eraCode, status) - 1;
     if (U_FAILURE(status)) {
         return;
     }
     if (uprv_add32_overflow(year, -startYear,  &year)) {
         status = U_ILLEGAL_ARGUMENT_ERROR;
         return;
     }
diff --git a/intl/icu/source/i18n/japancal.h b/intl/icu/source/i18n/japancal.h
--- a/intl/icu/source/i18n/japancal.h
+++ b/intl/icu/source/i18n/japancal.h
@@ -199,17 +199,17 @@ protected:
      * era, if the current month is an ascension year and month.
      * @param eyear the extended year
      * @param mon the month in the year
      * @param status Indicates the status.
      * @internal
      */
     virtual int32_t getDefaultDayInMonth(int32_t eyear, int32_t month, UErrorCode& status) override;
 
-    virtual bool isEra0CountingBackward() const override { return false; }
+    virtual bool isEra0CountingBackward() const override { return true; }
 };
 
 U_NAMESPACE_END
 
 #endif /* #if !UCONFIG_NO_FORMATTING */
 
 #endif
 //eof