This pattern seems to satisfy what you're looking for:
>>> pattern = re.compile('.*?(\d\d/\d\d/\d\d \d\d?:\d\d\w\w)')
>>> pattern.match('here is a lot of text that i dont need')
>>> pattern.match('this is the span i need because it contains 04/18/13 7:29pm').groups()
('04/18/13 7:29pm',)