from datetime import datetime
import pytz


def to_iso_time(timestamp):
    """Receives an arbitrary timestamp in UTC format (most likely in unix timestamp) and returns it as ISO-format.

    :param timestamp: arbitrary timestamp
    :return: timestamp in ISO 8601 and UTC timezone
    """
    if isinstance(timestamp, (int, float)):
        return datetime.utcfromtimestamp(timestamp).replace(tzinfo=pytz.UTC).isoformat()
    if timestamp is None:
        return datetime.utcnow().replace(tzinfo=pytz.UTC).isoformat()
    return timestamp