Check again when they arrived

The note on the visa can be over-written by INS officer at entry. When your parents arrive and go through the INS process at customs, they will assign them a final time period. It canbe the same as the note, and it can also be longer than it. Besides, filing an extension is also possible, and based on efficiency of INS office, maybe you will not get approval letter within 6 months, and during the waiting period, your parents will be legal.