import time


def ut2iso(ut, sep=' '):
    """UTC UT (rounded seconds) -> 'YYYY-MM-DD HH:MM:SS'. No TZ or offset."""
    tm = time.gmtime(round(ut))
    return '%04d-%02d-%02d%s%02d:%02d:%02d' % \
        (tm.tm_year, tm.tm_mon, tm.tm_mday, sep, tm.tm_hour, tm.tm_min, tm.tm_sec)