'Returns the timestamp that should be used if ``now`` is the current
time and ``last`` is the last timestamp returned by this object.
Intended for internal and testing use only; to generate timestamps,
call an instantiated ``MonotonicTimestampGenerator`` object.
:param int now: an integer to be used as the current time, typically
representing the current time in seconds since the UNIX epoch
:param int last: an integer representing the last timestamp returned by
this object'

def _next_timestamp(self, now, last):

    if (now > last):
        self.last = now
        return now
    else:
        self._maybe_warn(now=now)
        self.last = (last + 1)
        return self.last
