The bottling year is indicated for most bottles, where this is not the case, you can sometimes figure out when a bottle was actually bottled from the bottling code found on some bottles of whisky.
However many codes only mean something to the bottler and were put on the bottles for quality control reasons. A few examples:
A good example of bottling codes found on malt whisky bottles are those found on whiskies bottled by Glenmorangie. Do note that the bottling codes they use can be easily interpreted.
For example the bottle a Ardbeg has a code very faintly engraved into the glass near the bottom of the bottle. An example of such a code is: L7 256 21:15 4ML This code translated means it was bottled in the year 2007 on the 256th day at 9.15pm that day. 4ML is the code for Ardbeg.
Diageo has a similar and easy system, for example Talisker has the following code printed just above the rear label on the glass. An example of such a code is: L7302CM000. This code translated means it was bottled in the year 2007 on the 302nd day and it is a Classic Malt. There is another line of numbers underneath this code were put on the bottle by Diageo for quality control reasons.