From The Digital Picture
Bryan and Norbert over at TDP think they have deciphered the date codes built into Canon's newish 10 digital serial numbers. Previous to July 2010, Canon put date codes on all lenses other than EF-S that let you know when the lens was manufactured.
To age a Canon lens using the 10-digital serial number Canon lenses, we dissect the serial number as:
DD C SSSSSSS
The DD is the key to the date of manufacture of the lens – the date code. The long, narrow Canon lens date code chart is shown on the right side of this page.
They have posted what they predict to be the future date codes for Canon lenses. Please keep in mind that this is a work in progress and may not yet be definitive.