How to change django datetime format output?

I have a datetime object. This is my template file:

<ul>
<li>{{ sale.validity }}</li>
</ul>

and the output i'm getting is in the format:

July 18, 2012, midnight

I would really like to change the output to be numerical in the format: day-month-year so for example the above would be changed to:

18-07-2012

Can someone point me out to the right direction on this?