Many people consider September 7, 1998, the day Google was incorporated, to be it’s birthday. Google has a birthday logo on its home page today as well as a blog post about the the company’s birthday.
When is the born on date of a startup? When you first file papers of incorporation? When you move into your first company-specific office? When you hire your first employee? When you close your first deal?
I consider the birth of a startup to be the first time you solve the problem you set out to solve. It might be the first time your software did what it was supposed to do, or the first time you closed a sale if you are a services business. What do you think?