Determine the start and end dates based on user-supplied options.

def _metrics_get_endpoints(options):
    """ Determine the start and end dates based on user-supplied options. """
    if bool(options.start) ^ bool(options.end):
        log.error('--start and --end must be specified together')
        sys.exit(1)

    if options.start and options.end:
        start = options.start
        end = options.end
    else:
        end = datetime.utcnow()
        start = end - timedelta(options.days)

    return start, end