test(bigquery): Add tests to verify lossless max timestamp parsing

java-bigquery/google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/FieldValueTest.java

@@ -151,6 +151,18 @@ public void testInt64Timestamp() {
     assertEquals(expected, received);
   }
 
+  @Test
+  public void testLosslessMaxTimestamp() {
+    // Test the lossless behavior (useInt64Timestamps = true)
+    // The backend returns a 64-bit integer string for microseconds when this option is enabled
+    String losslessTimestampString = "253402300799999999";
+    FieldValue losslessValue =
+        FieldValue.of(FieldValue.Attribute.PRIMITIVE, losslessTimestampString, true);
+
+    // Exactly matches the microsecond equivalent of 9999-12-31 23:59:59.999999
+    assertEquals(253402300799999999L, losslessValue.getTimestampValue());
+  }
+
   @Test
   public void testEquals() {
     FieldValue booleanValue = FieldValue.of(FieldValue.Attribute.PRIMITIVE, "false");

java-bigquery/google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/it/ITBigQueryTest.java

@@ -1215,6 +1215,56 @@ static GoogleCredentials loadCredentials(String credentialFile) {
     }
   }
 
+  @Test
+  void testLosslessMaxTimestampIntegration() throws InterruptedException {
+    String query = "SELECT TIMESTAMP '9999-12-31 23:59:59.999999 UTC' as max_ts";
+    QueryJobConfiguration config = QueryJobConfiguration.newBuilder(query).build();
+
+    // 1. Test lossless 64-bit integer parsing (useInt64Timestamps = true)
+    DataFormatOptions losslessOptions =
+        DataFormatOptions.newBuilder().useInt64Timestamp(true).build();
+    BigQuery losslessBigQuery =
+        bigquery.getOptions().toBuilder()
+            .setDataFormatOptions(losslessOptions)
+            .build()
+            .getService();
+
+    TableResult losslessResult = losslessBigQuery.query(config);
+    assertEquals(1L, losslessResult.getTotalRows());
+    for (FieldValueList row : losslessResult.iterateAll()) {
+      long exactMicros = row.get("max_ts").getTimestampValue();
+      assertEquals(253402300799999999L, exactMicros);
+    }
+
+    // 2. Test lossy FLOAT64 rounding behavior (useInt64Timestamps = false)
+    DataFormatOptions floatOptions =
+        DataFormatOptions.newBuilder().useInt64Timestamp(false).build();
+    BigQuery floatBigQuery =
+        bigquery.getOptions().toBuilder().setDataFormatOptions(floatOptions).build().getService();
+
+    TableResult floatResult = floatBigQuery.query(config);
+    assertEquals(1L, floatResult.getTotalRows());
+    for (FieldValueList row : floatResult.iterateAll()) {
+      long roundedMicros = row.get("max_ts").getTimestampValue();
+      assertEquals(253402300800000000L, roundedMicros);
+    }
+
+    // 3. Test ISO8601 timestamp formatting
+    DataFormatOptions isoOptions =
+        DataFormatOptions.newBuilder()
+            .timestampFormatOptions(DataFormatOptions.TimestampFormatOptions.ISO8601_STRING)
+            .build();
+    BigQuery isoBigQuery =
+        bigquery.getOptions().toBuilder().setDataFormatOptions(isoOptions).build().getService();
+
+    TableResult isoResult = isoBigQuery.query(config);
+    assertEquals(1L, isoResult.getTotalRows());
+    for (FieldValueList row : isoResult.iterateAll()) {
+      String isoValue = row.get("max_ts").getStringValue();
+      assertEquals("9999-12-31T23:59:59.999999Z", isoValue);
+    }
+  }
+
   @Test
   void testListDatasets() {
     Page<Dataset> datasets = bigquery.listDatasets("bigquery-public-data");