Django models, adding new value, migrations
As long as your migration isn't applied to the database you can manually update your migration file located in myapp/migrations/*.py. Find the string '10.07.2016' and update it to a supported format.
A less attractive solution would be to delete the old migration file (as long as it isn't apllied to the database) and create a new migrations file with python manage.py makemigrations. Because you've updated the model to use a default value it won't ask for a one-off default this time.
To check whether a migration is applied to the database run: python manage.py showmigrations.
I worked with django 1.9 and added a new field (creation_date) to myapp/models.py.
After that I run "python manage.py makemigrations". I got: 

Please select a fix:

Provide a one-off default now (will be set on all existing rows)
Quit, and let me add a default in models.py."


I choose 1-st option and added value in wrong format '10.07.2016'.
After this mistake I couldn't run "python manage.py migrate".
So I decided to change models.py and add a default value "datetime.now".
But after that I still have problems with "python manage.py makemigrations". I see such things like that:

django.core.exceptions.ValidationError: [u"'10.07.2016' value has an invalid format. It must be in YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ] format."]

How to solve this problem?