X00 Subfield $y - Chronological subdivision:

Subfield $y contains a subject subdivision that represents a period of time. Subfield $y is appropriate in the X00 fields only when a chronological subject subdivision is added to a name or name/title heading to form an extended subject heading .

Example:

100 1#$aShakespeare, William,$d1564-1616$xCriticism and interpretation$xHistory$y18th century