Variable subtraction in django templates

You need to use double quotes:

{{ myval|add:"-5" }}

This subtracts five from myval.

The built-in Django template tags/filters aren't all-encompassing, but it's super easy to write your own custom template tags: https://docs.djangoproject.com/en/dev/howto/custom-template-tags/

You could make your own subtract template tag pretty easily:

@register.filter
def subtract(value, arg):
    return value - arg