Here's a solution using pytz module:
from pytz import timezone
from datetime import datetime
eastern = timezone('US/Eastern')
utc = timezone('UTC')
created_at = datetime.strptime(tweet[0]['created_at'], '%a %b %d %H:%M:%S +0000 %Y')
utc_created_at = utc.localize(created_at)
print utc_created_at
est_created_at = utc_created_at.astimezone(eastern)
print est_created_at
prints:
2013-06-18 22:23:22+00:00
2013-06-18 18:23:22-04:00
Hope that helps.