Omitting same "month" in date result

Hi all,

How can i change the my output that prints ( 15th March 2013 to 16th March 2013 ) into ( 15th - 16th March 2013 )
and from ( 15th March 2013 to 16th April 2013 ) becomes ( 15th March - 16th April 2013 ) ?

i've been trying to do something on the template.php but just couldnt figure out how ...

thanks a whole lot !