This is the McCann Lighting Company's date code decoder chart for their linear, U-bend, and circline fluorescent lamps. PL flourescents, HIDs, ballasts, fixtures, and pretty much everything else uses the 4-digit code system (2K13 is the format and 2K13 is this week's date code. It means the 2nd week of October 2013.)
for the 4-digit code system, a week counts as a full 7 days. If a week is split into two months, whatever month the Sundat of that week is, is the month. For example, September 2013 ended on a Monday, but since that week's Sunday was in September, that whole week is comsidered to be September. The date code for October 3rd 2013 would still be 5J13 even though it's October and not September.