You can also e-mail topglock or glockmeister with your serial number and they will be able to give you a "born on" date, usually. There are some serial numbers they don't have.
The born on date that we are using on GT is the date on the case collection envelopes. Although it may not truly be the day it was manufactured, it is the day that the first rounds were fired through the gun, semi-officially bringing it to life.