Medications have an expiration date. Meaning if past expiration date they possibly lose their potency and are not going to be as effective, if effective at all. Each medication varies on their expirate date. Typically you'll see prescribed medications good for approximately 1 year. (I'm uncertain the exact timeframe for nitroglycerin but, you could contact any pharmacy and the pharmacist should be able to tell you.)
Also, with nitro, it is important to be stored properly. It should be stored away from heat, moisture, and light. If not stored properly that can effect the effectiveness of the drug also. I can tell you, that I'm pretty definate medications from 2006 will be expired and need to thrown away.
If your parents see a family doctor atleast once a year, I'm almost certain the doctor wouldn't have a problem refilling the medication. I encourage you to visit with your parents further to find out the reasoning behind why they are not keeping this medication current. Maybe there just needs to be further educating.
I carry nitro with me all the time, but never had to use it yet.
I go by the expiration date on the bottle, but that's only cause I don't ever open the bottle.
Some say if you open the bottle breaking the seal, that because of moister in the air, the pills will only be good for about 3 months if 3 months is earlier then the expiration date.
I wish they could do something better with people like me, like make a smaller bottle of maybe only 5 to 10 pills, and maybe just automatically mail news ones to me ever 6 months. It's a pain to remember to keep on top of them and the doctors never inquire about them during the visits.