anchor.diff:      };
       if (values.length > 0) {
         ret['values'] = new Map.fromIterable(values.keys,
  -                                           key: (DateTime date) => date.toIso8601String(),
  +                                           key: (DateTime date) => date.toUtc().toIso8601String(),
                                              value: (DateTime date) => values[date]);
       }
       return ret;
  
positive.code:       pollutant = location.pollutants.lookup(pollutant);
      }
  
      DateTime date = timezone.TZDateTime.parse(warsaw, item.findElements('Date').single.text);
      var value = double.parse(item.findElements('Value').single.text.replaceAll(',', '.'));
      pollutant.values[date] = value;
      if (pollutant.lastValue == null || date.isAfter(pollutant.lastValue.dateTime)) {
  
negative.code: 
    PollutantValue(DateTime this.dateTime, double this.value);
  
    toJson() => {'dateTime': dateTime.toUtc().toIso8601String(), 'value': value};
  }
  
  class Pollutant {
  