The serial number stamped on the fuselage gives you the date the fuselage was built at the factory. The data plate is stamped and installed by the builder.

Example is my kitfox. Serial #C9408 0047 ie the 47th Classic 4 fuselage produced at the factory in 1994 and completed in August. On My data plate I stamped a manufacture date of May 2000, the date I completed building it and a maximum gross weight of 1050 due to my 503 Rotax engine.