With Sam Gawith, about all you can use is the presence or absence of the red color used on the label. If there is red color, they are post 2005 IIRC.
Mac Baren uses a 10 (?) didit code on the back in some cases and in others, the date simply expressed like a date. With the former, the third and forth digits are the year. The next three are the day expressed as a number 1-365. C&D and GLP uses a date like 010115 (Jan 1 2015) McClelland uses a six digit code where the last two are the year.
Mike S.