If it's just a curiosity issue, I would feel comfortable that the first 4 digits are the year of manufacture, but the next four may be month and day or possibly batch number. My three EN-EL15 batteries conform to a year, month and day format because they start with 2010 or 2012, the next two digits are less than 12 and the remaining two digits are less than 31.
If you're looking at battery age as an indicator of remaining battery quality you should probably look at the remaining battery life rather than the oldest. Newer batteries may have less remaining life than older ones based on how they were used, stored, number of charges, etc. You can check remaining battery life by checking Battery Information on the D7000 menu, if you're interested.
"Red is gray and yellow white, but we decide which is right ....and which is an illusion"