def expires(duration, vary=None, currtime=time.time):
    """Decorator. Apply on a :class:`wsgiservice.Resource` method to set the
    max-age cache control parameter to the given duration. Also calculates
    the correct ``Expires`` response header.

    :param duration: Age which this resource may have before becoming stale.
    :type duration: :mod:`datetime.timedelta`
    :param vary: List of headers that should be added to the Vary response
                 header.
    :type vary: list of strings
    :param currtime: Function used to find out the current UTC time. This is
                     used for testing and not required in production code.
    :type currtime: Function returning a :mod:`time.struct_time`
    """
    if isinstance(duration, timedelta):
        duration = timedelta_to_seconds(duration)

    @decorator
    def _expires(func, *args, **kwargs):
        "Sets the expirations header to the given duration."
        res = args[0].response

        res.cache_control.max_age = duration
        res.expires = currtime() + duration

        if vary:
            if res.vary is None:
                res.vary = vary
            else:
                # A bit completed because res.vary is usually a tuple.
                res.vary = list(set(list(res.vary) + list(vary)))

        return func(*args, **kwargs)
    return _expires