code: def to_timestamp(arg, format_str, timezone=None):
      
      return ops.StringToTimestamp(arg, format_str, timezone).to_expr()
docs: Parses a string and returns a timestamp.
  
      Parameters
      ----------
      format_str : A format string potentially of the type '%Y-%m-%d'
      timezone : An optional string indicating the timezone,
          i.e. 'America/New_York'
  
      Examples
      --------
      >>> import ibis
      >>> date_as_str = ibis.literal('20170206')
      >>> result = date_as_str.to_timestamp('%Y%m%d')
  
      Returns
      -------
      parsed : TimestampValue