User blog comment:Icehard712/Nerd fight, anyone?/@comment-11643042-20150214044412

I believe I know the exact date of Dipper and Mabel's birthday!

First of all, the series takes place in 2012. Dipper and Mabel are known to be 12 years old during the series. And Alex has said that their birthday coincides with the end of summer.

Now, we know from Dipper's bag in the theme song that he and Mabel live in Piedmont, California. According to my research, Piedmont's school systems began on August 22, 2012. Therefore, allowing 2 days for travel, I would place Dipper and Mabel's birthday on August 19, 1999.