Parse the date as you are already doing, and then
date= date.replace(2013)
This is one of simplest solution with the modules you are using.
Thinking better about it, you will probably face a problem next Feb 29.
input= 'Aug21 3:26PM'
output= datetime.datetime.strptime('2013 '+ input ,'%Y %b%d %I:%M%p')