The vintage date on wine bottles refers to the date the grapes were picked, not when the wine was bottled.
Vintage refers to the year the grapes were harvested. It does not refer to the date the wine was bottled or released to market. For a wine to have a year printed on the bottle, the wine must be made from 95% of one year’s harvest.
This is how Canada, the U.S., and Bordeaux, France bottles, anyways. If there is no vintage date, then one can assume that the grapes in the wine came from several harvests. Other places in France and the rest of the world are more relaxed on their vintage dates.
So, how do you even know when to drink a vintage wine? Most producers will give you a good idea as to when the wine’s “peak” date to drink it is. The date also helps wine connoisseurs to pin down which harvests and wines they like the best.
They can then get more information on the harvest, such as weather patterns, different styles or techniques used by the producer, or any diseases or problems during that harvest.