Outputing period (duration) from a template seems to be rather complex.
Also, 'pretty' format with words 'ago', 'in' may be a problem for
outputing duration only .

I create some functions for simplifying this task:
HTMLDate
def periodof(self, pretty=1, othdate='.'):
    """ Render the duration between this date and other date (default is
now). If the "pretty" flag is true, then make the display human readable."""

Interval
def period()
# return interval in human readable format such as "1 month", "2 week" etc.

Use it in templates as:
issue.closedate.periodof(othdate = issue.creation)