code: def to_utc_timestamp_mills(self, epoch_mills):
      if self._shift_timezone == "UTC":
          return epoch_mills
      else:
          timezone = pytz.timezone(self._shift_timezone)
          local_date_time = datetime.datetime.fromtimestamp(
              epoch_mills / 1000.0, timezone
          ).replace(tzinfo=None)
          epoch = datetime.datetime.utcfromtimestamp(0)
          return int((local_date_time - epoch).total_seconds() * 1000.0)
  
imports: import datetime
  from abc import ABC, abstractmethod
  from typing import TypeVar, Generic, List, Dict
  import pytz
  from pyflink.common import Row, RowKind
  from pyflink.common.constants import MAX_LONG_VALUE
  from pyflink.fn_execution.datastream.timerservice import InternalTimer
  from pyflink.fn_execution.datastream.process.timerservice_impl import LegacyInternalTimerServiceImpl
  from pyflink.fn_execution.coders import PickleCoder
  from pyflink.fn_execution.table.aggregate_slow import DistinctViewDescriptor, RowKeySelector
  from pyflink.fn_execution.table.state_data_view import DataViewSpec, ListViewSpec, MapViewSpec, PerWindowStateDataViewStore
  from pyflink.fn_execution.table.window_assigner import WindowAssigner, PanedWindowAssigner, MergingWindowAssigner
  from pyflink.fn_execution.table.window_context import WindowContext, TriggerContext, K, W
  from pyflink.fn_execution.table.window_process_function import GeneralWindowProcessFunction, InternalWindowProcessFunction, PanedWindowProcessFunction, MergingWindowProcessFunction
  from pyflink.fn_execution.table.window_trigger import Trigger
  from pyflink.table.udf import ImperativeAggregateFunction, FunctionContext
label_desc: Use image node_id 8 for calling the GroupWindowAggFunctionBase obj's underlying member method code with example usage: obj.to_utc_timestamp_mills(epoch_mills) and returns: epoch_mills, int