Tires: Expiry date of tires????, 4 digits, cold climate

Question
The tires on my car were purchased & installed in January 2002. The tread is still in reasonably good condition, not down to the "wear bars" yet. Is there some sort of rule or guidelines as to how long the rubber in the tires is good without being a hazard?
I really appreciate any advise you can offer regarding this.
Thanks!

Answer
John,

There have been many recent bulletins on the subject, but the answer is far from clear.

My take on all this is that if you live in a hot climate (AZ, CA, NV, NM, TX and FL), then 6 years is the limit (from date of manufacture) and if you live in a cold climate (MN, MT, WI, ND, etc.) the limit is 10 years.  If you live in a state in between ... ah .... Mmmmmm ..... it's in between.

To determine the date of manufacture, look for the letters "DOT".  There will be a 10 to 12 digit number. (Note:  You may have to look at both sides of the tire to get the complete number) The last 3 or 4 digits are the date in a week/week/year or week/week/year/year format, with tires produced 1999 and earlier having 3 digits and tires produced 2001 and newer having 4 digits and there is a transition in 1999 and 2000 where both 3 and 4 digits were produced.