The termina date determination can be split into "terminus ante que" and "terminus post que", which are the comparative analyses to find the latest and earliest possible dates, respectively, on papyrus scrolls. By using such a comparison - when available - the margin of error on one end of the dating is effectively eliminated, leaving just the +25 years or -25 years.
Usage by Historians
Archaeologists may discover manuscripts at an excavation that could not have been written past a certain date or due to a natural disaster (Mt. Vesuvius in AD 79), abandonment of the site (Qumrum caves in AD 70). Since no one was left to continue writing, then those "terminus ante que" dates are known.
Usage for Papyrologists
Remember from the earlier chapter that papyrus scrolls were written in the recto side, since the reeds would form a horizontal pattern to aid in text alignment. Once completed, the scroll would be rolled, and the text would be kept safe on the inside. Due to the expense of papyrus, the recipients of the scrolls would frequently reuse the surface on the verso for a number of different needs.
Of the surviving papyri manuscripts (not just New Testament), this is the case on numerous occasions. For example, New Testament text may be on the recto and work by Homer on the verso, or a land contract may be on the recto and New Testament text on the verso. If the date of one side could be determined, then that would help to date the other side.
Since calendars were not the same in ancient times as they are now, the manuscript with a date would not say something as simple as "written AD 179". It would be written as "in the third year of ..." with reference to an emperor, an on-going war, the age of a known person, etc. By converting the ancient calendar to the modern one, that date establishes the termina date for one side of the papyrus.
If the manuscript with a known date was written on the recto, then it could logically be stated that the verso side was written later. Likewise, if the dated manuscript was written on the verso side, then the recto text must have been written earlier.
By @HullBreach
