python - Django's ORM. Group by elements with difference -

i have django model:

class visits(models.model):     ip = models.genericipaddressfield()     url = models.charfield(max_length=200)     datetime = models.datetimefield(auto_now_add=true)   def __unicode__(self):     return "%s : %s (%s)" % (unicode(self.ip),                               unicode(self.datetime.strftime('%d/%m/%y %h:%m:%s')),                               unicode(self.url)) 

it contains visits website. example, these values db:

<visits: 127.0.0.1 : 27/11/14 00:00:00 (/)>, <visits: 127.0.0.1 : 27/11/14 00:01:00 (/)>,  <visits: 127.0.0.1 : 27/11/14 00:09:00 (/)>,  <visits: 127.0.0.1 : 27/11/14 00:45:00 (/)>,  <visits: 127.0.0.1 : 27/11/14 00:46:00 (/)> 

i want write orm query groups values 15-minutes delta. example, query should return count()==2 (first 3 values in first group, second 2 in second). how can using django's orm?

here have 4 visits , tried filter out minitues grater 28.

>>> visits_count = len([ obj obj in visits.objects.all() if obj.datetime.strftime('%m') >  '28']) >>> visits_count 2 

it case minitues greater equal 28.

>>> visits_count = len([ obj obj in visits.objects.all() if obj.datetime.strftime('%m') >=  '28']) >>> visits_count 4