Most Plantinga products have had a production code on their caps, at least since 1991 but probably already at least 10 years earlier.
During UTO times (until 2004) the code could be deciphered as follows:

PDDDYQQ

P = Production line nummer
DDD = Xth day of the year
Y = Last digit of the year (which unfortunately allows for easy confusion between 198x, 199x and 200x)
QQ = Xth quarter of the day

When production went to Boomsma, the code consisted of two lines and could be deciphered as follows:

LYYBBBB

DDD UU:MM

L = Always an "L",  indicating that it's a grocery item
YY = Last two digits of the year
BBBB = Batch number
DDD = Xth day of the year
HH:MM = Production time in hours and minutes