$ [sudo] pip install django-timedelta
INSTALLED_APPS+=['django_timedelta']
{% load timedelta %}
{% if started_at and completed_at %}
{% timedelta completed_at started_at %}
{% timedelta completed_at started_at 3 %}
{% endif %}
0:00:42.123456
{% timedelta completed_at started_at 3 %}
0:00:42.123