def _NextMaintenanceToCell(zone):
  """Returns the start time of the next maintenance or ''."""
  maintenance_events = zone.get('maintenanceWindows', [])
  if maintenance_events:
    next_event = min(maintenance_events, key=lambda x: x.get('beginTime'))
    return next_event.get('beginTime') + '--' + next_event.get('endTime')
  else:
    return ''