Canon has been putting a date code on its lenses since the 1960’s, but ever since 2008 the company has been transitioning to a 10-digit serial system, sans date code. At first this caused quite a stir, as many a conspiracy theory began popping up; but not to worry, the code is there, it’s just been incorporated into the new 10-digit system.
You can find the full guide to dating your Canon lens — be it using the old or new systems — here. For now the codes have held up, but given the new system, March of 2013 will either start over at 00 or move to a three digit date, displacing the batch number, shortening the serial, or lengthening the whole shebang. So keep an eye out, dating your Canon lens is about to get interesting… or at least as interesting as dating a lens could possibly get.
Determining the Age of a Canon Lens [The Digital Picture]