Not certain about the unit you have, but I bought a propane
gas detector and noted that the date on the back of it indicated it was made six months earlier. I questioned the retailer and she phoned the distributor who contacted the manufacturer.
The manufacturer said the clock starts running from the date it is connected to an electrical
So you're probably good for a couple more years than you think. Also, mine indicates with a chirp and LED that it is nearing end of life.
I'd check the installation manual that came with your unit, or look it up online if you didn't get the manual.
I'd also vacuum the face plate or wipe it with a damp cloth as indicated in the manual.