Why does json serialization of datetime objects in python not work out of the box for datetime objects

No it doesn’t work that way in json module. The module provides you with a default encoder: json.JSONEncoder. You need to extend this to provide your implementation of default method to serialize objects. Something like this:

import json
import datetime
from time import mktime

class MyEncoder(json.JSONEncoder):

    def default(self, obj):
        if isinstance(obj, datetime.datetime):
            return int(mktime(obj.timetuple()))

        return json.JSONEncoder.default(self, obj)

print json.dumps(obj, cls=MyEncoder)

As others correctly pointed out, the reason is that the standard for json does not specify how date time can be represented.