If the date of copyright can be found, use that, and place a ‘c’ in front the date, e.g. c1955. If you are able to estimate or infer the date, enter the year followed by a ‘?’ in square brackets, e.g. [1999?]. If no date can be found or estimated, use [date unknown].
Examples:
Brookfield S. Understanding and facilitating adult learning: a comprehensive analysis of principles and effective practices. Buckingham: Open University Press; 1998.
Drake RL, Vogl W, Mitchell AM, Gray H. Gray's anatomy for students. 2nd ed. Philadelphia: Churchill Livingstone/Elsevier; 2010.
Culpeper N. Culpeper's astrological judgment of diseases. [London?]: [publisher unknown]; [date unknown].