What is an exceptable DOT date code for new tires??

I guess we were very fortunate. When we got our used coach in 2010 the tires were the OEM ones from the chassis build date of 2001. The first thing I did after getting it home was to go to our local Schwab store. Without me even asking, the store manager checked the warehouse inventory by manufacture date and found a set that were only 6 weeks old,