In 1986, with the design of the EOS System (Electro-Optical System, EOS started to ship in 1987), Canon started added a date code to their lenses.
Many of the new Canon lenses do not have a date code.
A large number of the Canon lenses have a date code in the form of "UX0311". Often it's on the lens mount.
The first letter, "U", indicates that the lens was made in Utsunomiya factory.
U = Utsunomiya, Japan
F = Fukushima, Japan
O = Oita, Japan