There are a lot of attempts in this thread to make things black or white. For example, your health can change in a year, so what good is a 11-month old signature? Yes, that is certainly true. But the caveat that goes along with the "valid for 12 months" standard is "if nothing has changed." If a students hands me a doctor's signature, I am expecting that the student's health is as it was at the time of the signature, and may well ask if it is. "No changes in the 10 months since this was signed?" At some point, you have to trust the student, just as the student has to trust the instructor to do a good job. But the standards in place do not extend that trust to all situations and all time. 12 months max for a signature, if nothing has changed, and the hope that the student has actually talked to the doctor honestly too.