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ManyToMany relationship in django models

I'm trying to add relation between three table with manyToMany relationship. I'm learning python and new for Django framework. For requirement, An user can add a project and he also can add permission for view for another user. It means an user can have multiple projects and a project also have multiple user. User model is default user model of django.
and the project model is:
Projects/model.py
 from django.db import models  
 from django.contrib.auth.models import User  
 # Create your models here.  
 class Projects(models.Model):  
   project_title = models.CharField(max_length=255)  
   description = models.TextField(blank=True)  
   privacy = models.SmallIntegerField(default=1) # 1 for public, 2 for private  
   status = models.SmallIntegerField(default=1) #1 for active, 2 for deactive, 3 for delete, 4 for depricated  
   created_at = models.DateTimeField('date published', blank=True, default=False)  
   users = models.ManyToManyField(User, through="UserProjects")  
 class UserProjects(models.Model):  
   user = models.ForeignKey(User,blank=True, default=False)  
   project = models.ForeignKey(Projects,blank=True)  
   owner = models.SmallIntegerField(default=1)

It's not working. When i'm add a project is produce some error..
 Traceback (most recent call last):  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\core\handlers\exception.py", line 41, in inner  
   response = get_response(request)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\core\handlers\base.py", line 187, in _get_response  
   response = self.process_exception_by_middleware(e, request)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\core\handlers\base.py", line 185, in _get_response  
   response = wrapped_callback(request, *callback_args, **callback_kwargs)  
  File "E:\Programs\Python\RestClient\Projects\views.py", line 29, in add  
   p.save()  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\base.py", line 806, in save  
   force_update=force_update, update_fields=update_fields)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\base.py", line 836, in save_base  
   updated = self._save_table(raw, cls, force_insert, force_update, using, upda  
 te_fields)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\base.py", line 922, in _save_table  
   result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\base.py", line 961, in _do_insert  
   using=using, raw=raw)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\manager.py", line 85, in manager_method  
   return getattr(self.get_queryset(), name)(*args, **kwargs)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\query.py", line 1063, in _insert  
   return query.get_compiler(using=using).execute_sql(return_id)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\sql\compiler.py", line 1098, in execute_sql  
   for sql, params in self.as_sql():  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\sql\compiler.py", line 1051, in as_sql  
   for obj in self.query.objs  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\sql\compiler.py", line 1051, in <listcomp>  
   for obj in self.query.objs  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\sql\compiler.py", line 1050, in <listcomp>  
   [self.prepare_value(field, self.pre_save_val(field, obj)) for field in field  
 s]  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\sql\compiler.py", line 990, in prepare_value  
   value = field.get_db_prep_save(value, connection=self.connection)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\fields\__init__.py", line 770, in get_db_prep_save  
   prepared=False)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\fields\__init__.py", line 1459, in get_db_prep_value  
   value = self.get_prep_value(value)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\fields\__init__.py", line 1438, in get_prep_value  
   value = super(DateTimeField, self).get_prep_value(value)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\fields\__init__.py", line 1296, in get_prep_value  
   return self.to_python(value)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\db\models\fields\__init__.py", line 1399, in to_python  
   parsed = parse_datetime(value)  
  File "C:\Users\Jitendra\AppData\Local\Programs\Python\Python35-32\lib\site-pac  
 kages\django\utils\dateparse.py", line 94, in parse_datetime  
   match = datetime_re.match(value)  
 TypeError: expected string or bytes-like object  
 [28/Jul/2017 22:25:10] "POST /projects/add/ HTTP/1.1" 500 145532


And here the projects/view.py

 def add(request):  
   form = ProjectForm()  
   if request.method == "POST":  
     form = ProjectForm(request.POST)  
     if form.is_valid():  
       p = Projects(project_title=request.POST.get("project_title"), description=request.POST.get("description"),  
              status=1, privacy=1)  
       p.save()  
       sp = UserProjects(user_id=request.user.id, project_id=p.id)  
       sp.save()  
       p.users.add(sp)  
       messages.success(request, 'Profile details updated.')  
   view_data = {  
     "form": form,  
     'title': "Add new project"  
   }  
   return render(request, 'projects/add.html', view_data)

I dont know, whats wrong with it. Please guide me. Thanks
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