old_contents: from django.db import models
  
  from . import fields
  
  
  class ReducedDateStartAndEndMixin(models.Model):
      class Meta:
          abstract = True
  
      start_date = fields.ReducedDateField()
      end_date = fields.ReducedDateField()
  
  
  class TimeTrackingMixin(models.Model):
      """
      Mixin for adding a created_at and updated_at field
      """
      class Meta:
          abstract = True
  
      created_at = models.DateTimeField(auto_now=True)
      updated_at = models.DateTimeField(auto_now_add=True)
  
new_contents: from django.db import models
  
  from . import fields
  
  
  class ReducedDateStartAndEndMixin(models.Model):
      """
      Provides a ``start_date`` and ``end_date`` field for a given model
  
      This is a common set of fields found throughout Popolo models.
      """
      class Meta:
          abstract = True
  
      start_date = fields.ReducedDateField()
      end_date = fields.ReducedDateField()
  
  
  class TimeTrackingMixin(models.Model):
      """
      Mixin for adding a ``created_at`` and ``updated_at`` field
  
      Most entities inside Popolo provide an auto-generated ``created_at``
      and ``updated_at`` time stamp.
      """
      class Meta:
          abstract = True
  
      created_at = models.DateTimeField(auto_now=True)
      updated_at = models.DateTimeField(auto_now_add=True)
  
subject: Add in docblocks for mixins