The dates on USAF/US Army serials are the Fiscal Year the funds were appropriated. In some years where there is heavy aircraft production, such as WWII, FY and manufacturing year will usually be the same. However, often funds are appropriated quite a few years before the actual first flight of a type, so that the first model can be not even close to the same year. Then others can be authorized once a type is selected, and the production starts. All the planes paid for in that FY may take a while to leave the production line.
For the most part, that FY becomes a "circa" year, but it's the best we have without having the actual data plate or other information from the plane. I think having it there for a "circa" date is good.
Now, if only there was a way to know the years for the Navy/USMC/USCG numbers!