Since 2000, the week and year the tire was produced has been provided by the last four digits of the Tire Identification Number (TIN) with the 2 digits being used to identify the week immediately preceding the 2 digits used to identify the year.
Example of a tire manufactured since 2000 with the current Tire Identification Number format:
In the example above:
DOT H2LF YA9J 3507
35—Manufactured during the 35th week of the year
07—Manufactured during 2007
If you don't visibly see the DOT tire code with the date code from the outside view, then you'll need to view the other side of the tire.
The Tire Identification Number for tires produced prior to 2000 was based on the assumption that tires would not be in service for ten years. While they were required to provide the same information as today's tires, the week and year the tire was produced was contained in the last three digits. The 2 digits used to identify the week a tire was manufactured immediately preceded a single digit used to identify the year.
Example of a tire manufactured before 2000 with the earlier Tire Identification Number format:
In the example above:
17—Manufactured during the 17th week of the year
9—Manufactured during 1999 or could have been 1989 or...
According to the The Tire Safety Group and the NTSB Tire Safety Symposium (PDF), it is recommended that all tires older than six years should be replaced regardless of the tread depth or appearance. Another excellent resource is this article, How to Know When to Replace Tires.
Due to trailer suspension design, there are unique forces such as "Interply Shear" placed on multi-axle trailer tires. This unique force causes about 24% higher shear forces than seen on motorized vehicles. This means you would need to run a decreased load (maybe 25% to 50% less) to get the same life on a multi-axle trailer application than the same tire on a tow vehicle application."
Roger does not know of any direct comparison in real life testing, so, he can only guess at the above figures, other than the 24% that comes from Finite Element computer simulation which is a well developed tool in automotive circles other than the RV industry.
You can read more about this on Roger's blog.
The tire manufacturer is assigned a tire plant code that forms the first two digits of the DOT code. In the example picture below, the plant code is AT. The tire was manufactured in England.
Common chart for truck and trailer tires.
The load index number on the tire indicates the maximum load that tire was designed for.
The speed rating letter on the tire indicates the maximum speed that tire was designed for.