Oops... Sorry! They're 3415 and 3515, ie a week apart August of last year.
From a couple of the articles I read, it sounds like a year to 18 months or so isn't that atypical for less-common sizes of certain tires. Motorcycle tires can be even older.
Apparently, for service interval, you use the "in-service" date of the tire if known, and revert to the tire date *only if you don't know the actual date the tire went in service*.
Unless somebody knows something more, I think it's OK.
SC