Within the template, no, you cannot import python code.
The way to do this is to register the function as a jinja2 custom filter, like this:
In your python file:
from dates.format import timesince
environment = jinja2.Environment(whatever)
environment.filters['timesince'] = timesince
# render template here
In your template:
{% macro time(mytime) %}
<a title="{{ mytime }}">{{ mytime|timesince }}</a>
{% endmacro %}