code: def get_timestamp_expression(self, time_grain):
          
          label = utils.DTTM_ALIAS
  
          db = self.table.database
          pdf = self.python_date_format
          is_epoch = pdf in ('epoch_s', 'epoch_ms')
          if not self.expression and not time_grain and not is_epoch:
              sqla_col = column(self.column_name, type_=DateTime)
              return self.table.make_sqla_column_compatible(sqla_col, label)
          grain = None
          if time_grain:
              grain = db.grains_dict().get(time_grain)
              if not grain:
                  raise NotImplementedError(
                      f'No grain spec for {time_grain} for database {db.database_name}')
          col = db.db_engine_spec.get_timestamp_column(self.expression, self.column_name)
          expr = db.db_engine_spec.get_time_expr(col, pdf, time_grain, grain)
          sqla_col = literal_column(expr, type_=DateTime)
          return self.table.make_sqla_column_compatible(sqla_col, label)
docs: Getting the time component of the query