Convert dates to strings

json-date.py

import json
from datetime import datetime

class Foo(object):

   def __init__(self, when):
       self.when = when

   def __json__(self):
       return self.__dict__

def dumps(obj):
    return json.dumps(obj, default=convert)

def convert(obj):
    if callable(getattr(obj, '__json__', None)):
        return obj.__json__()
    if isinstance(obj, datetime):
        return str(obj)
    raise TypeError("Cannot convert", obj)

print dumps(Foo(datetime.now()))