# Don't use Gannen year numbering for Gregorian eras in Japanese calendar.

diff --git a/intl/icu/source/i18n/smpdtfmt.cpp b/intl/icu/source/i18n/smpdtfmt.cpp
--- a/intl/icu/source/i18n/smpdtfmt.cpp
+++ b/intl/icu/source/i18n/smpdtfmt.cpp
@@ -1066,16 +1066,64 @@ SimpleDateFormat::_format(Calendar& cal,
             calClone->setTimeZone(cal.getTimeZone());
             workCal = calClone;
         } else {
             status = U_MEMORY_ALLOCATION_ERROR;
             return appendTo;
         }
     }
 
+    // Set or reset Gannen year numbering.
+    if (typeid(*fCalendar) == typeid(JapaneseCalendar) && fHasHanYearChar &&
+        uprv_strcmp(fLocale.getLanguage(), "ja") == 0) {
+
+        int32_t era = workCal->get(UCAL_ERA, status);
+        if (U_FAILURE(status)) {
+            return appendTo;
+        }
+
+        // Cast away constness.
+        auto* self = const_cast<SimpleDateFormat *>(this);
+
+        // Does the current calendar era uses Gannen year numbering?
+        bool useGannen = era != GregorianCalendar::AD && era != GregorianCalendar::BC;
+
+        // Code to set/reset numbering copied from `SimpleDateFormat::applyPattern`.
+        if (!useGannen && fDateOverride == UnicodeString(u"y=jpanyear")) {
+            // Gannen numbering is set but current era should not use it, unset;
+            // use procedure from adoptNumberFormat to clear overrides
+            if (fSharedNumberFormatters) {
+                freeSharedNumberFormatters(fSharedNumberFormatters);
+                self->fSharedNumberFormatters = nullptr;
+            }
+            self->fDateOverride.setToBogus(); // record status
+        } else if (useGannen && fDateOverride.isBogus()) {
+            // No current override (=> no Gannen numbering) but current era needs it;
+            // use procedures from initNUmberFormatters / adoptNumberFormat
+            umtx_lock(&LOCK);
+            if (fSharedNumberFormatters == nullptr) {
+                self->fSharedNumberFormatters = allocSharedNumberFormatters();
+            }
+            umtx_unlock(&LOCK);
+            if (fSharedNumberFormatters != nullptr) {
+                Locale ovrLoc(fLocale.getLanguage(), fLocale.getCountry(), fLocale.getVariant(), "numbers=jpanyear");
+                const SharedNumberFormat *snf = createSharedNumberFormat(ovrLoc, status);
+                if (U_FAILURE(status)) {
+                    return appendTo;
+                }
+                // Now that we have an appropriate number formatter, fill in the
+                // appropriate slot in the number formatters table.
+                UDateFormatField patternCharIndex = DateFormatSymbols::getPatternCharIndex(u'y');
+                SharedObject::copyPtr(snf, fSharedNumberFormatters[patternCharIndex]);
+                snf->deleteIfZeroRefCount();
+                self->fDateOverride.setTo(u"y=jpanyear", -1); // record status
+            }
+        }
+    }
+
     UBool inQuote = false;
     char16_t prevCh = 0;
     int32_t count = 0;
     int32_t fieldNum = 0;
     UDisplayContext capitalizationContext = getContext(UDISPCTX_TYPE_CAPITALIZATION, status);
 
     // loop through the pattern string character by character
     int32_t patternLength = fPattern.length();