First off, depite what Dalton says, beer is food. Therefore, it has a shelf life. Sure, some beers age gracefully, but most age VERY poorly. Generally speaking, the lighter the beer, the lousier it ages.
Corona does NOT age well. It has a brewery-recommended shelf life of 9 months from the time it is bottled. That code on the bottle neck is the bottling date... real consumer-friendly isn't it?!
Simply put, the first letter at the beginning of the code corresponds to the year. For example:
5 = 2005
6 = 2006
and so on. This is the year in which the beer was bottled.
The second number refers to the month in which it was bottleed, but in reverse. For example:
A = December
B = November
C = October
and so on, skipping the letter "I" because it looks too much like the number "1".
The next 2 numbers should be looked at as a calendar date. For example:
02 = 2nd day of the month
23 = 23rd day of the month
etc.
To sum up, a bottle with the code date
"GA07" would be a production date of 2007, December 07.
This beer is just over 4 months old and still technically good.
A code that reads
"GM19" would be a production date of 2007, January 19.
This beer is out-of-code and will not taste "brewery-fresh"... whatever that means to Corona.
Corona is so cheaply-made and simple to begin with, as long as it is kept out of the light and cold, it will stay "good" for a long time.
Clear glass is more susceptible to light-damage than brown, which is why Corona is served with a lime - to cover up the "skunky" flavors.
It also has plenty of preservatives in it.
Keep in mind that NO harmful bacteria can live in beer, unless that beer is left open and uncovered for a long period of time. Sealed, the beer is safe from all contaminants that could be detrimental to your health...
At most it will taste bad... which Corona already does unless you add lime to it.
Cheers.