From http://www.thegritty.com/birthdaycelebrations:
Rule #1:
Your birthday is defined as 11AM-12 Midnight on the day of your birthday. Bring an I.D. (driver's license). If you are a child bring a trustworthy parent or grandparent.
Rule #2:
Drink free soda, specialty birthday cocktails or beer on your birthday and keep the "memento" mug. For those last minute gifts, buy yourself or a friend a Gritty t-shirt, a birthday cake, a Gritty sparkler or a Gritty glo-lite birthday button!
This is absolutely NOT true. I visited the Nitty briefly in the early afternoon of my birthday to buy a drink and was told that they're policy is to provide free drinks on your birthday. At first it sounded too good to be true. But sure enough I got a free beer, which was all I wanted to drink at the time.
However, I assumed after looking up these rules that I could return in the evening since it was still my birthday. However, I was informed that after you leave the Nitty, that is treated as the end of your birthday.
So beware, free beer is only offered during a single visit. If you decide to come back later, you're SOL.
Thanks to the management for making me PAY for my birthday beer at the so-called "birthday place". I will not be returning and would suggest the same for any friend who's birthday I want to celebrate the right way.