That means that the play was without fail written after (in Latin, post) 1587.
Relative dating methods are unable to determine the absolute age of an object or event, but can determine the impossibility of a particular event happening before or after another event of which the absolute date is well known.
Historians, for example, know that Shakespeare's play Henry V was not written before 1587 because Shakespeare's primary source for writing his play was the second edition of Raphael Holinshed's Chronicles, not published until 1587.
Thus, 1587 is the post quem dating of Shakespeare's play Henry V.
When this hydrated layer or rind reaches a thickness of about 0.5 microns, it becomes recognizable as a birefringent rim when observed as a thin section under a microscope.
Hydration rims formed on artifacts can vary in width from less than one micron for items from the early historic period to nearly 30 microns for early sites in Africa (Michels et al. Formation of the hydration rim is affected not only by time but also by several other variables.