Use the archival date abbreviation

Warning²Federal courts, Illinois courts22nd edition only

An archival date parenthetical carries two fixed abbreviations: “n.d.” where the document gives no readable date, and the prefix “c.” where the date is only approximate.¹

1. 1R. 23.5 Date.
2. 2A warning: the form is usually wrong under the rule in note 1, and context can justify it.