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diff --git a/java/com/google/gerrit/server/notedb/CommentTimestampAdapter.java b/java/com/google/gerrit/server/notedb/CommentTimestampAdapter.java
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+// Copyright (C) 2017 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.notedb;
+
+import static java.time.format.DateTimeFormatter.ISO_INSTANT;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.sql.Timestamp;
+import java.time.Instant;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+import java.time.format.DateTimeFormatter;
+import java.time.format.DateTimeParseException;
+import java.time.format.FormatStyle;
+import java.time.temporal.TemporalAccessor;
+
+/**
+ * Adapter that reads/writes {@link Timestamp}s as ISO 8601 instant in UTC.
+ *
+ * <p>This adapter reads and writes the ISO 8601 UTC instant format, {@code "2015-06-22T17:11:00Z"}.
+ * This format is specially chosen because it is also readable by the default Gson type adapter,
+ * despite the fact that the default adapter writes in a different format lacking timezones, {@code
+ * "Jun 22, 2015 10:11:00 AM"}. Unlike the default adapter format, this representation is not
+ * ambiguous during the transition away from DST.
+ *
+ * <p>This adapter is mutually compatible with the old adapter: the old adapter is able to read the
+ * UTC instant format, and this adapter can fall back to parsing the old format.
+ *
+ * <p>Older Gson versions are not able to parse milliseconds out of ISO 8601 instants, so this
+ * implementation truncates to seconds when writing. This is no worse than the truncation that
+ * happens to fit NoteDb timestamps into git commit formatting.
+ */
+class CommentTimestampAdapter extends TypeAdapter<Timestamp> {
+  private static final DateTimeFormatter FALLBACK =
+      DateTimeFormatter.ofLocalizedDateTime(FormatStyle.MEDIUM);
+
+  @Override
+  public void write(JsonWriter out, Timestamp ts) throws IOException {
+    Timestamp truncated = new Timestamp(ts.getTime() / 1000 * 1000);
+    out.value(ISO_INSTANT.format(truncated.toInstant()));
+  }
+
+  @Override
+  public Timestamp read(JsonReader in) throws IOException {
+    String str = in.nextString();
+    TemporalAccessor ta;
+    try {
+      ta = ISO_INSTANT.parse(str);
+    } catch (DateTimeParseException e) {
+      ta = LocalDateTime.from(FALLBACK.parse(str)).atZone(ZoneId.systemDefault());
+    }
+    return Timestamp.from(Instant.from(ta));
+  }
+}