diff --git a/native/jni/src/suggest/policyimpl/dictionary/bigram/ver4_bigram_list_policy.cpp b/native/jni/src/suggest/policyimpl/dictionary/bigram/ver4_bigram_list_policy.cpp
index ad437b1..968bace 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/bigram/ver4_bigram_list_policy.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/bigram/ver4_bigram_list_policy.cpp
@@ -153,12 +153,16 @@
                 return false;
             }
         } else if (mNeedsToDecayWhenUpdating) {
-            // TODO: Quit decaying probability during GC.
             const int probability = ForgettingCurveUtils::getEncodedProbabilityToSave(
                     bigramEntry.getProbability(), mHeaderPolicy);
+            const HistoricalInfo historicalInfo =
+                    ForgettingCurveUtils::createHistoricalInfoToSave(
+                            bigramEntry.getHistoricalInfo());
+            // TODO: Use ForgettingCurveUtils::needsToKeep(&historicalInfo).
             if (ForgettingCurveUtils::isValidEncodedProbability(probability)) {
                 const BigramEntry updatedBigramEntry =
-                        bigramEntry.updateProbabilityAndGetEntry(probability);
+                        bigramEntry.updateProbabilityAndGetEntry(probability)
+                                .updateHistoricalInfoAndGetEntry(&historicalInfo);
                 if (!mBigramDictContent->writeBigramEntry(&updatedBigramEntry, entryPos)) {
                     return false;
                 }
@@ -225,7 +229,7 @@
         const int probability = ForgettingCurveUtils::getUpdatedEncodedProbability(
                 originalBigramEntry->getProbability(), newProbability);
         const HistoricalInfo updatedHistoricalInfo =
-                ForgettingCurveUtils::createUpdatedHistoricalInfoFrom(
+                ForgettingCurveUtils::createUpdatedHistoricalInfo(
                         originalBigramEntry->getHistoricalInfo(), newProbability, timestamp);
         return originalBigramEntry->updateProbabilityAndGetEntry(probability)
                 .updateHistoricalInfoAndGetEntry(&updatedHistoricalInfo);
diff --git a/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_patricia_trie_node_writer.cpp b/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_patricia_trie_node_writer.cpp
index 867cd76..0755434 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_patricia_trie_node_writer.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/structure/v4/ver4_patricia_trie_node_writer.cpp
@@ -151,17 +151,22 @@
         const ProbabilityEntry originalProbabilityEntry =
                 mBuffers->getProbabilityDictContent()->getProbabilityEntry(
                         toBeUpdatedPtNodeParams->getTerminalId());
-        // TODO: Use historical info.
+        // TODO: Remove.
         const int newProbability = ForgettingCurveUtils::getEncodedProbabilityToSave(
                 originalProbabilityEntry.getProbability(), mBuffers->getHeaderPolicy());
+        const HistoricalInfo historicalInfo =
+                ForgettingCurveUtils::createHistoricalInfoToSave(
+                        originalProbabilityEntry.getHistoricalInfo());
         const ProbabilityEntry probabilityEntry =
-                originalProbabilityEntry.createEntryWithUpdatedProbability(newProbability);
+                originalProbabilityEntry.createEntryWithUpdatedProbability(newProbability)
+                        .createEntryWithUpdatedHistoricalInfo(&historicalInfo);
         if (!mBuffers->getMutableProbabilityDictContent()->setProbabilityEntry(
                 toBeUpdatedPtNodeParams->getTerminalId(), &probabilityEntry)) {
             AKLOGE("Cannot write updated probability entry. terminalId: %d",
                     toBeUpdatedPtNodeParams->getTerminalId());
             return false;
         }
+        // TODO: Use ForgettingCurveUtils::needsToKeep(&historicalInfo).
         const bool isValid = ForgettingCurveUtils::isValidEncodedProbability(newProbability);
         if (!isValid) {
             if (!markPtNodeAsWillBecomeNonTerminal(toBeUpdatedPtNodeParams)) {
@@ -379,7 +384,7 @@
         const int updatedProbability = ForgettingCurveUtils::getUpdatedEncodedProbability(
                 originalProbabilityEntry->getProbability(), newProbability);
         const HistoricalInfo updatedHistoricalInfo =
-                ForgettingCurveUtils::createUpdatedHistoricalInfoFrom(
+                ForgettingCurveUtils::createUpdatedHistoricalInfo(
                         originalProbabilityEntry->getHistoricalInfo(), newProbability, timestamp);
         return originalProbabilityEntry->createEntryWithUpdatedProbability(updatedProbability)
                 .createEntryWithUpdatedHistoricalInfo(&updatedHistoricalInfo);
diff --git a/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.cpp b/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.cpp
index c7fb47e..466af72 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.cpp
@@ -42,10 +42,13 @@
 const int ForgettingCurveUtils::MAX_LEVEL = 3;
 const int ForgettingCurveUtils::MAX_COUNT = 3;
 const int ForgettingCurveUtils::MIN_VALID_LEVEL = 1;
+const int ForgettingCurveUtils::TIME_STEP_DURATION_IN_SECONDS = 6 * 60 * 60;
+const int ForgettingCurveUtils::MAX_ELAPSED_TIME_STEP_COUNT = 15;
+const int ForgettingCurveUtils::DISCARD_LEVEL_ZERO_ENTRY_TIME_STEP_COUNT_THRESHOLD = 14;
 
 const ForgettingCurveUtils::ProbabilityTable ForgettingCurveUtils::sProbabilityTable;
 
-/* static */ const HistoricalInfo ForgettingCurveUtils::createUpdatedHistoricalInfoFrom(
+/* static */ const HistoricalInfo ForgettingCurveUtils::createUpdatedHistoricalInfo(
         const HistoricalInfo *const originalHistoricalInfo,
         const int newProbability, const int timestamp) {
     if (newProbability != NOT_A_PROBABILITY && originalHistoricalInfo->getLevel() == 0) {
@@ -110,6 +113,12 @@
     return encodedProbability >= MIN_VALID_ENCODED_PROBABILITY;
 }
 
+/* static */ bool ForgettingCurveUtils::needsToKeep(const HistoricalInfo *const historicalInfo) {
+    return historicalInfo->getLevel() > 0
+            || getElapsedTimeStepCount(historicalInfo->getTimeStamp())
+                    < DISCARD_LEVEL_ZERO_ENTRY_TIME_STEP_COUNT_THRESHOLD;
+}
+
 /* static */ int ForgettingCurveUtils::getEncodedProbabilityToSave(const int encodedProbability,
         const DictionaryHeaderStructurePolicy *const headerPolicy) {
     const int elapsedTime = TimeKeeper::peekCurrentTime() - headerPolicy->getLastDecayedTime();
@@ -129,6 +138,26 @@
     return currentEncodedProbability;
 }
 
+/* static */ const HistoricalInfo ForgettingCurveUtils::createHistoricalInfoToSave(
+        const HistoricalInfo *const originalHistoricalInfo) {
+    if (originalHistoricalInfo->getTimeStamp() == NOT_A_TIMESTAMP) {
+        return HistoricalInfo();
+    }
+    const int elapsedTimeStep = getElapsedTimeStepCount(originalHistoricalInfo->getTimeStamp());
+    if (elapsedTimeStep < MAX_ELAPSED_TIME_STEP_COUNT) {
+        // No need to update historical info.
+        return *originalHistoricalInfo;
+    }
+    // Level down.
+    const int maxLevelDownAmonut = elapsedTimeStep / MAX_ELAPSED_TIME_STEP_COUNT;
+    const int levelDownAmount = (maxLevelDownAmonut >= originalHistoricalInfo->getLevel()) ?
+            originalHistoricalInfo->getLevel() : maxLevelDownAmonut;
+    const int adjustedTimestamp = originalHistoricalInfo->getTimeStamp() +
+            levelDownAmount * MAX_ELAPSED_TIME_STEP_COUNT * TIME_STEP_DURATION_IN_SECONDS;
+    return HistoricalInfo(adjustedTimestamp,
+            originalHistoricalInfo->getLevel() - levelDownAmount, 0 /* count */);
+}
+
 /* static */ bool ForgettingCurveUtils::needsToDecay(const bool mindsBlockByDecay,
         const int unigramCount, const int bigramCount,
         const DictionaryHeaderStructurePolicy *const headerPolicy) {
@@ -167,6 +196,10 @@
     }
 }
 
+/* static */ int ForgettingCurveUtils::getElapsedTimeStepCount(const int timestamp) {
+    return (TimeKeeper::peekCurrentTime() - timestamp) / TIME_STEP_DURATION_IN_SECONDS;
+}
+
 ForgettingCurveUtils::ProbabilityTable::ProbabilityTable() : mTable() {
     // Table entry is as follows:
     // 1, 1, 1, 2, 3, 5, 6, 9, 13, 18, 25, 34, 48, 66, 91, 127.
diff --git a/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.h b/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.h
index 3e5bdf6..76d172e 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/utils/forgetting_curve_utils.h
@@ -36,18 +36,26 @@
     static const int MAX_BIGRAM_COUNT;
     static const int MAX_BIGRAM_COUNT_AFTER_GC;
 
-    static const HistoricalInfo createUpdatedHistoricalInfoFrom(
+    static const HistoricalInfo createUpdatedHistoricalInfo(
             const HistoricalInfo *const originalHistoricalInfo, const int newProbability,
             const int timestamp);
 
+    static const HistoricalInfo createHistoricalInfoToSave(
+            const HistoricalInfo *const originalHistoricalInfo);
+
     static int getProbability(const int encodedUnigramProbability,
             const int encodedBigramProbability);
 
+    // TODO: Remove.
     static int getUpdatedEncodedProbability(const int originalEncodedProbability,
             const int newProbability);
 
+    // TODO: Remove.
     static int isValidEncodedProbability(const int encodedProbability);
 
+    static bool needsToKeep(const HistoricalInfo *const historicalInfo);
+
+    // TODO: Remove.
     static int getEncodedProbabilityToSave(const int encodedProbability,
             const DictionaryHeaderStructurePolicy *const headerPolicy);
 
@@ -84,12 +92,17 @@
     static const int MAX_LEVEL;
     static const int MAX_COUNT;
     static const int MIN_VALID_LEVEL;
+    static const int TIME_STEP_DURATION_IN_SECONDS;
+    static const int MAX_ELAPSED_TIME_STEP_COUNT;
+    static const int DISCARD_LEVEL_ZERO_ENTRY_TIME_STEP_COUNT_THRESHOLD;
 
     static const ProbabilityTable sProbabilityTable;
 
     static int decodeProbability(const int encodedProbability);
 
     static int backoff(const int unigramProbability);
+
+    static int getElapsedTimeStepCount(const int timestamp);
 };
 } // namespace latinime
 #endif /* LATINIME_FORGETTING_CURVE_UTILS_H */