Date code of a lens

How can I find out the date code of 28-135mm IS USM?

On the rear of the Lens, you'll find the code, as per:
http://www.the-digital-picture.com …ses/Canon-Lens-Aging.aspx

that will be very hard because the 28-135IS doesn't have this date code on the rear.

Is it just the L lenses which have a date code?

I believe all L lenses do have date codes, but the 35f2 also has it. The 50f1.4 and 85f1.8 do not.

some of the older EF lenses also have date codes.

My 10-22mm EF-S doesn't have a date code, neither does my 75-300 IS USM.
Both of my L's do have dates codes, and both were made in Sept. 2005 at Utsunomiya.