Many things in aviation have a 60-day window. For example, a graduation certificate from an approved part 141 course is valid for the purposes of a checkride for 60 days after the date of graduation.
Suppose I endorse an (otherwise fully qualified) student to take a private pilot knowledge test, but the student doesn't take the test for an extended period (because of illness, military deployment, or whatever). Some amount of time later, the student appears at a testing center with the endorsement in hand. Is there a particular time limit beyond which a knowledge test endorsement is no longer valid?
If not, could I write the endorsement so that it includes a void date? Something like:
I certify that Mr. Orville Wright has met the required training of blah blah and is prepared to take the private pilot knowledge exam. This endorsement is valid until October 31 2017.