from datetime import datetime


def TimestampUTCToISO8601(timestamp):
  """Return the timestamp (UTC) ISO 8601 format: YYYY-MM-DD."""
  utc_tuple = datetime.utcfromtimestamp(timestamp)
  return '%0.4d-%0.2d-%0.2d' % (utc_tuple.year, utc_tuple.month, utc_tuple.day)