Django SQLite UPDATE

I am developing a webapp using django. Generally, I want to keep track of how much time a user has devoted to a project. So when the user starts working, I am registering a "Change" object in the database with the user, the project, and what time he start working. Now, how can I get the same "Change" object to a different view to log the end time? In my best attempt I got an "IntegrityError NOT NULL constraint failed". Here is the code:

views.py

@login_required
def main(request):
    context = RequestContext(request)
    user=request.user
    if request.method=='POST':
        projectid=request.POST['project']
        project = Project.objects.get(pk=int(projectid))
        if project:
            change=Change(user=user, project=project,starttime=datetime.now())
            change.save()
            #return HttpResponse("I \'ve already start counting... You \'ve wasted 5 seconds already. Just joking. I don\'t know how to count...")
            return render_to_response('ProjectLogging/timer.html', {'change':change}, context_instance=RequestContext(request))
        else:
            HttpResponse("Choose a valid project!")
    else:
        HttpResponse("Choose a POST method (???????)")

@login_required
def timer(request, change):
    user=request.user
    # schange=request.change
    if request.method=='POST':
        change=Change(endtime=datetime.now())
        change.save()
        return render_to_response('ProjectLogging/main.html',{'user':user, 'project_list':Project.objects.all()}, context_instance=RequestContext(request))
    else:
        return HttpResponse("Something went wrong with timer view.")

urls.py

urlpatterns=patterns('ProjectLogging',
    url(r'^$', 'views.login', name="index"),
    url(r'^login/$', 'views.login', name="login"),
    url(r'^logout/$', 'views.logout', name="logout"),
    url(r'^main/$','views.main', name="main"),
    url(r'^timer/$', 'views.timer', {'change':'change'}),
)

and the Change model

class Change(models.Model):
    user=models.ForeignKey(User)
    project=models.ForeignKey('Project')
    starttime=models.DateTimeField(null=True,blank=True)
    endtime=models.DateTimeField(null=True, blank=True)
    flagged=models.BooleanField(default=False, db_index=True)

    def __repr__(self):
        return ('Change in project %r from %r' % (self.project.title, self.user.username))

    def __unicode__(self):
        return ('%r in project: %r' % (self.user.get_full_name(), self.project.title))

    def user_changes(self, user):
        return self.filter(user==user)

Sorry for the primitive code. I m new to both Django and Python.