def _offset_to_tzname(offset):
    """
    Converts an offset in minutes to an RFC 3339 "time-offset" string.

    >>> _offset_to_tzname(0)
    '+00:00'
    >>> _offset_to_tzname(-1)
    '-00:01'
    >>> _offset_to_tzname(-60)
    '-01:00'
    >>> _offset_to_tzname(-779)
    '-12:59'
    >>> _offset_to_tzname(1)
    '+00:01'
    >>> _offset_to_tzname(60)
    '+01:00'
    >>> _offset_to_tzname(779)
    '+12:59'
    """
    offset = int(offset)
    if offset < 0:
        tzsign = '-'
    else:
        tzsign = '+'
    offset = abs(offset)
    tzhour = offset / 60
    tzmin = offset % 60
    return '%s%02d:%02d' % (tzsign, tzhour, tzmin)