Maxi-Cosi Priori Expiration Date?

That is probably the date of manufacture. I would call the company and confirm.

Call the company to confirm, but most car seats "expire" 5 years after their date of manufacture.

Yes car seats "expire". Sadly it probably is a ploy to sell more products and limit reselling of carseats/passing them on to friends and family to help save us money. With that said I do know the thought is that over time in a hot car as well as all sorts of temperature changes and sun exposure, the plastic degrades. If the plastic is compromised, then potentially the said plastic could not hold up in a crash. I believe the general life of a car seat is 10 years. But don't quote me on that. Check out your car seats manufacturers website, I know Britex should have some information on this issue on their website.

I would check the company website to see if they tell you where to find the seat expiration. I know that Graco has a page with pictures of where to locate the manufacturing info and expiration date. If you don't find it on their site, then they certainly must have a customer service number listed that you can call to ask. I hope the seat is still useable. :)
http://www.maxi-cosi.com/us-en

No worries... the date should be Manufactured date... not the expiration..... I don't think expiration dates are really set in stone, and definitely no something the manufacturer wants to be responsible for....
I believe the general rule of thumb is 6 years from the date on the carseat. ... but seats that sit in a hot car baking in the sun all day, should be sooner because the plastic could be compromised by extreme heating and cooling. ... Which is why people are so leary of a used carseat.. the integrity of the seat materials (plastics really) are questionable.

But back to your question....
Your carseat's registration website tells you where the Manufacture date is... and it does sound just like what you describe..
http://www.maxi-cosi.com/us-en/register

I have never heard of a car seat 'expiring'.... It doesn't really 'go bad'. You are suppose to replace car seats that have been used after a certain time period or which are significantly outdated. The website below says that you should add 5-8 years to the DOM date in order to determine when the seat should no longer be used.

From: http://www.carseat.org/Technical/tech_update.htm#expdate

How old is too old for a safety seat?
There is some controversy about the "expiration" date for safety seats. All experts agree that a seat should be discarded and destroyed if it is more than 10 years old, even if it looks fine. Most manufacturers suggest replacing a seat 5 to 8 years after the date of manufacture, because current safety seats may have better safety features than older seats, such as a tether or air bag warnings. The date of manufacture may be found on a sticker on the seat (unless it has peeled off) and may be stamped into the plastic shell. However, don’t confuse patent dates, which can also be molded into the plastic, with the date the individual safety seat was manufactured.

Expiration date
In the last few years, CR manufacturers have been putting "expiration dates" on their products. This was begun in response to infant deaths from air bags. The companies realized that they had millions of products in use that said nothing about the danger to a rear-facing child. The concern became one that "best practice" and regulations change over time, so that a child restraint becomes "obsolete" and less effective than a new product, much the same way that medicines may change or become less effective after some time has passed. A current example would be with tethers--older CRs do not have them, and this is potentially a situation where a newer product will be more effective than an older one.

Taken at face value, the expiration interval (ranging from 5 to 8 years, depending on the manufacturer) is from the date of manufacture, which is what governs the labeling, certification, and other rules that apply. Whether to continue to use an "expired" CR is a judgment call, depending on the alternatives. It will work as well as before, but it is important to know what regulations and warnings may have changed and the risks involved. It is also important to take the expiration date into account when purchasing a CR that may have been manufactured several months or even a year or more before.

On all our carseats & booster seats the experation date is molded into the plastic not on the the label. If you bought the car seat new (not from a private party) then I am sure that your carseat is still good. But you can always call the manufacturer and ask them where to find the experation date.

After sitting in the hot sun in the hot car over time the plastic becomes weaker.

You're looking at the date of manufacture. This is needed in case there is a recall for certain dates, or any other date issue. Car seats don't expire. Like everything in this world, they are making newer "better" ones all the time, so it might become out-of-date. Just like a computer program becomes out-of-date when they make a newer version. You can still use the older version forever. I still use Windows 98.

I have heard it said that you should replace your car seat after 6 years. I have had my origional one now for 5 years and am expecting to use it for number three due in January. I will, of course, have it checked by the CHP first as always.