8.4.3. Maintenance exam - time of reference. Maintenance Exams should address a time of
reference subsequent to the date of conviction or the previous Maintenance Exam,
generally not exceeding one (1) year and only exceeding two (2) years in rare
circumstances. The time of reference may be described generally as the six (6) month
period preceding the examination; although, there may be reasons for lengthening or
shortening the time of reference for some exams. All investigation targets in a test series
should have a common time of reference.

Doing research I found this guideline from the A.P.A., which required by most states (including my own), polygraphers must follow by, and have went to school accredited by the A.P.A.. 

My next questions: In what rare circumstances must they go 2+ years?
Also their has got to be a reason why they have a Time Of Reference.. Is it because the polygraph loses its effectiveness?