old_contents: from django.db import models
  import datetime
  
  class Timer(models.Model):
      name = models.CharField(max_length=200)
      start_time = models.DateTimeField()
      duration = models.IntegerField(null=True)
      running = models.NullBooleanField(default=True)
      stopped_at = models.DateTimeField(null=True)
  
      @property
      def end_time(self):
          if self.duration:
              return self.start_time + datetime.timedelta(seconds=self.duration)
          return datetime.datetime.now()
  
new_contents: from django.db import models
  from django.utils.timezone import now
  
  class Timer(models.Model):
      name = models.CharField(max_length=200)
      start_time = models.DateTimeField()
      duration = models.IntegerField(null=True)
      running = models.NullBooleanField(default=True)
      stopped_at = models.DateTimeField(null=True)
  
      @property
      def end_time(self):
          if self.duration:
              return self.start_time + datetime.timedelta(seconds=self.duration)
          return now()
  
subject: Use timezone-aware datetime