There's a manufacturer's shipping code on almost every automobile battery. Since the life you'll get from the battery is dependent on when it was made, not necessarily when it was put into service, you'll want to check this code so you can be sure the battery you buy is relatively "fresh" and hasn't been sitting on the shelf for a couple of years. The shipping code usually appears on a sticker on the battery or a stamp burned into the battery case itself.
The code is a string of letters numbers, but all the real information you need is in the first two characters. Usually, the code starts with a letter that corresponds with the month--'A' for January through 'L' for December (some manufacturer's codes omit 'I'; for them, 'M' indicates December). The second character of the code is a numeral that stands for the year. Thus, a code starting 'A2' indicates that the battery was shipped in January 1992; 'B2' indicates February 1992, and so on. (Delco reverses the letter and the number; a Delco code starting with '2C' indicates the battery was shipped in March 1992.)