old_contents: # ------------------------------------------------------------------------
  # coding=utf-8
  # ------------------------------------------------------------------------
  """
  Track the modification date for pages.
  """
  
  from django.db import models
  from django.db.models.signals import pre_save
  from django.utils.translation import ugettext_lazy as _
  
  def pre_save_handler(sender, instance, **kwargs):
      """
      Intercept attempts to save and insert the current date and time into
      creation and modification date fields.
      """
      from datetime import datetime
  
      now = datetime.now()
      if instance.id is None:
          instance.creation_date = now
      instance.modification_date = now
  
  # ------------------------------------------------------------------------
  def dt_to_utc_timestamp(dt):
      from time import mktime, gmtime
      return mktime(gmtime(mktime(dt.utctimetuple())))
  
  def register(cls, admin_cls):
      cls.add_to_class('creation_date',     models.DateTimeField(_('creation date'),     null=True, editable=False))
      cls.add_to_class('modification_date', models.DateTimeField(_('modification date'), null=True, editable=False))
  
      if hasattr(cls, 'cache_key_components'):
          cls.cache_key_components.append(lambda page: page.modification_date and page.modification_date.strftime('%s'))
  
      if hasattr(cls, 'last_modified'):
          cls.last_modified = lambda p: dt_to_utc_dt(p.modification_date)
  
      pre_save.connect(pre_save_handler, sender=cls)
  
  # ------------------------------------------------------------------------
  def last_modified_response_processor(self, request, response):
      from django.utils.http import http_date
  
      # Don't include Last-Modified if we don't want to be cached
      if "no-cache" in response.get('Cache-Control', ''):
          return
  
      # If we already have a Last-Modified, take the later one
      from email.utils import parsedate_tz, mktime_tz
  
      last_modified = [ dt_to_utc_timestamp(self.modification_date) ]
      if response.has_header('Last-Modified'):
          last_modified.append(mktime_tz(parsedate_tz(response['Last-Modified'])))
  
      response['Last-Modified'] = http_date(max(last_modified))
  
  # ------------------------------------------------------------------------
  
new_contents: # ------------------------------------------------------------------------
  # coding=utf-8
  # ------------------------------------------------------------------------
  """
  Track the modification date for pages.
  """
  
  import os
  
  from email.utils import parsedate_tz, mktime_tz
  
  from django.db import models
  from django.db.models.signals import pre_save
  from django.utils.translation import ugettext_lazy as _
  
  try:
      import pytz
      local_tz = pytz.timezone(os.environ['TZ'])
  except ImportError:
      class NoTimezone(object):
          def localize(_, dt):
              return dt
      local_tz = NoTimezone()
  
  # ------------------------------------------------------------------------
  def pre_save_handler(sender, instance, **kwargs):
      """
      Intercept attempts to save and insert the current date and time into
      creation and modification date fields.
      """
      from datetime import datetime
  
      now = datetime.now()
      if instance.id is None:
          instance.creation_date = now
      instance.modification_date = now
  
  # ------------------------------------------------------------------------
  def dt_to_utc_timestamp(dt):
      from time import mktime
      return int(mktime(dt.timetuple()))
  
  def register(cls, admin_cls):
      cls.add_to_class('creation_date',     models.DateTimeField(_('creation date'),     null=True, editable=False))
      cls.add_to_class('modification_date', models.DateTimeField(_('modification date'), null=True, editable=False))
  
      if hasattr(cls, 'cache_key_components'):
          cls.cache_key_components.append(lambda page: page.modification_date and str(dt_to_utc_timestamp(page.modification_date)))
  
      cls.last_modified = lambda p: p.modification_date
  
      pre_save.connect(pre_save_handler, sender=cls)
  
  # ------------------------------------------------------------------------
  def last_modified_response_processor(page, request, response):
      from django.utils.http import http_date
  
      # Don't include Last-Modified if we don't want to be cached
      if "no-cache" in response.get('Cache-Control', ''):
          return
  
      # If we already have a Last-Modified, take the later one
      last_modified = dt_to_utc_timestamp(page.last_modified())
      if response.has_header('Last-Modified'):
          last_modified = max(last_modified, mktime_tz(parsedate_tz(response['Last-Modified'])))
  
      response['Last-Modified'] = http_date(last_modified)
  
  # ------------------------------------------------------------------------
  
subject: Correct handling of last-modified times.