diff --git a/framework-t/src/android/net/NetworkStatsCollection.java b/framework-t/src/android/net/NetworkStatsCollection.java
index 29ea772..6a1d2dd 100644
--- a/framework-t/src/android/net/NetworkStatsCollection.java
+++ b/framework-t/src/android/net/NetworkStatsCollection.java
@@ -865,6 +865,9 @@
          * Add association of the history with the specified key in this map.
          *
          * @param key The object used to identify a network, see {@link Key}.
+         *            If history already exists for this key, then the passed-in history is appended
+         *            to the previously-passed in history. The caller must ensure that the history
+         *            passed-in timestamps are greater than all previously-passed-in timestamps.
          * @param history {@link NetworkStatsHistory} instance associated to the given {@link Key}.
          * @return The builder object.
          */
@@ -874,9 +877,21 @@
             Objects.requireNonNull(key);
             Objects.requireNonNull(history);
             final List<Entry> historyEntries = history.getEntries();
+            final NetworkStatsHistory existing = mEntries.get(key);
 
+            final int size = historyEntries.size() + ((existing != null) ? existing.size() : 0);
             final NetworkStatsHistory.Builder historyBuilder =
-                    new NetworkStatsHistory.Builder(mBucketDurationMillis, historyEntries.size());
+                    new NetworkStatsHistory.Builder(mBucketDurationMillis, size);
+
+            // TODO: this simply appends the entries to any entries that were already present in
+            // the builder, which requires the caller to pass in entries in order. We might be
+            // able to do better with something like recordHistory.
+            if (existing != null) {
+                for (Entry entry : existing.getEntries()) {
+                    historyBuilder.addEntry(entry);
+                }
+            }
+
             for (Entry entry : historyEntries) {
                 historyBuilder.addEntry(entry);
             }
diff --git a/framework-t/src/android/net/NetworkStatsHistory.java b/framework-t/src/android/net/NetworkStatsHistory.java
index 60dad90..0ff9d96 100644
--- a/framework-t/src/android/net/NetworkStatsHistory.java
+++ b/framework-t/src/android/net/NetworkStatsHistory.java
@@ -1136,14 +1136,44 @@
             mOperations = new ArrayList<>(initialCapacity);
         }
 
+        private void addToElement(List<Long> list, int pos, long value) {
+            list.set(pos, list.get(pos) + value);
+        }
+
         /**
          * Add an {@link Entry} into the {@link NetworkStatsHistory} instance.
          *
-         * @param entry The target {@link Entry} object.
+         * @param entry The target {@link Entry} object. The entry timestamp must be greater than
+         *              that of any previously-added entry.
          * @return The builder object.
          */
         @NonNull
         public Builder addEntry(@NonNull Entry entry) {
+            final int lastBucket = mBucketStart.size() - 1;
+            final long lastBucketStart = (lastBucket != -1) ? mBucketStart.get(lastBucket) : 0;
+
+            // If last bucket has the same timestamp, modify it instead of adding another bucket.
+            // This allows callers to pass in the same bucket twice (e.g., to accumulate
+            // data over time), but still requires that entries must be sorted.
+            // The importer will do this in case a rotated file has the same timestamp as
+            // the previous file.
+            if (lastBucket != -1 && entry.bucketStart == lastBucketStart) {
+                addToElement(mActiveTime, lastBucket, entry.activeTime);
+                addToElement(mRxBytes, lastBucket, entry.rxBytes);
+                addToElement(mRxPackets, lastBucket, entry.rxPackets);
+                addToElement(mTxBytes, lastBucket, entry.txBytes);
+                addToElement(mTxPackets, lastBucket, entry.txPackets);
+                addToElement(mOperations, lastBucket, entry.operations);
+                return this;
+            }
+
+            // Inserting in the middle is prohibited for performance reasons.
+            if (entry.bucketStart <= lastBucketStart) {
+                throw new IllegalArgumentException("new bucket start " + entry.bucketStart
+                        + " must be greater than last bucket start " + lastBucketStart);
+            }
+
+            // Common case: add entries at the end of the list.
             mBucketStart.add(entry.bucketStart);
             mActiveTime.add(entry.activeTime);
             mRxBytes.add(entry.rxBytes);
diff --git a/tests/common/java/android/net/netstats/NetworkStatsHistoryTest.kt b/tests/common/java/android/net/netstats/NetworkStatsHistoryTest.kt
index c2654c5..f8e041a 100644
--- a/tests/common/java/android/net/netstats/NetworkStatsHistoryTest.kt
+++ b/tests/common/java/android/net/netstats/NetworkStatsHistoryTest.kt
@@ -27,6 +27,7 @@
 import org.junit.runner.RunWith
 import org.junit.runners.JUnit4
 import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
 
 @ConnectivityModuleTest
 @RunWith(JUnit4::class)
@@ -51,12 +52,22 @@
                 .build()
         statsSingle.assertEntriesEqual(entry1)
         assertEquals(DateUtils.HOUR_IN_MILLIS, statsSingle.bucketDuration)
+
+        // Verify the builder throws if the timestamp of added entry is not greater than
+        // that of any previously-added entry.
+        assertFailsWith(IllegalArgumentException::class) {
+            NetworkStatsHistory
+                    .Builder(DateUtils.SECOND_IN_MILLIS, /* initialCapacity */ 0)
+                    .addEntry(entry1).addEntry(entry2).addEntry(entry3)
+                    .build()
+        }
+
         val statsMultiple = NetworkStatsHistory
                 .Builder(DateUtils.SECOND_IN_MILLIS, /* initialCapacity */ 0)
-                .addEntry(entry1).addEntry(entry2).addEntry(entry3)
+                .addEntry(entry3).addEntry(entry1).addEntry(entry2)
                 .build()
         assertEquals(DateUtils.SECOND_IN_MILLIS, statsMultiple.bucketDuration)
-        statsMultiple.assertEntriesEqual(entry1, entry2, entry3)
+        statsMultiple.assertEntriesEqual(entry3, entry1, entry2)
     }
 
     fun NetworkStatsHistory.assertEntriesEqual(vararg entries: NetworkStatsHistory.Entry) {
diff --git a/tests/unit/java/android/net/netstats/NetworkStatsDataMigrationUtilsTest.kt b/tests/unit/java/android/net/netstats/NetworkStatsDataMigrationUtilsTest.kt
index 743d39e..aa5a246 100644
--- a/tests/unit/java/android/net/netstats/NetworkStatsDataMigrationUtilsTest.kt
+++ b/tests/unit/java/android/net/netstats/NetworkStatsDataMigrationUtilsTest.kt
@@ -61,14 +61,6 @@
         assertValues(builder.build(), 55, 1814302L, 21050L, 31001636L, 26152L)
     }
 
-    @Test
-    fun testMaybeReadLegacyUid() {
-        val builder = NetworkStatsCollection.Builder(BUCKET_DURATION_MS)
-        NetworkStatsDataMigrationUtils.readLegacyUid(builder,
-                getInputStreamForResource(R.raw.netstats_uid_v4), false /* taggedData */)
-        assertValues(builder.build(), 223, 106245210L, 710722L, 1130647496L, 1103989L)
-    }
-
     private fun assertValues(
         collection: NetworkStatsCollection,
         expectedSize: Int,