turns out timetuple was not passing timezone information. the correct way of converting a datetime.datetime object to a correct rfc-2822 compliant date string seems to be:

email.utils.formatdate(time.mktime(a.utctimetuple()) + 1e-6 * a.microsecond - time.timezone)

what a mess. if the above is indeed the right way to do this, is it possible to add the following function to the email.utils module?

def formatdatetime(dt_object):
    return email.utils.formatdate(time.mktime(dt_object.utctimetuple()) + 1e-6 * a.microsecond - time.timezone)

this works for datetime instances both with and without time zone information.

ps: i updated the code in the github link but not here.