So, I got my two sweet gerbils on Feb 7th and I wanted to be able to celebrate their birthdays. They're both sisters from what the pet store lady told me, and they were around three months old then. That means their birthday is around or on November 18th, 2016! Now I know to prepare near that day :)
I never actually was able to celebrate any of my pet's birthdays because nobody gave me an estimate or actual date to work the math from. The former owners of my dog only knew that she was born sometime in July, but they didn't know what day. And usually pet store owners don't know how old the animals are. I definitely prefer going to Petsmart instead of Petco, because Petsmart staff seem to have more knowledge/care more about the animals.