In the world of Buzz Lightyear of Star Command, it is difficult to make a precise timeline of events when we lack the details to do so. But there is evidence throughout the series of events taking place before the formation of the Galactic Alliance and the start of the series, as well as events that take place during it.
Present-day Valkyrans claim that before the Galactic Alliance, they ruled the galaxy's space ways, and traveled anywhere they wished without hindrance. This probably took place some time after Natron's rule and closer to the formation of the Galactic Alliance, though this is all mere speculation.
Galactic Alliance and Star Command
Just as a precise date for the Galactic Alliance's formation is unknown, we do not know when exactly Star Command was first created. However, Buzz Lightyear, who is at least forty years old, had wanted to become a Space Ranger ever since he was young. Space Rangers are considered very popular amongst children, even in the present day.
Commander Nebula has also been Star Command's commanding officer for at least twenty years, his appearance barely having changed since then. However, given his age and him still missing a leg during Buzz's Academy years, he has certainly been at Star Command for much longer. Before being stranded behind a desk, Nebula was fairly active during his younger years.
Tangea is also one of the most recent planets to become a member of the Galactic Alliance, this having occurred during Mira's teenage years when Buzz saved her planet from the Evil Emperor Zurg. In fact, during the conference leading up to attempting to encourage Tangea to join, one of Buzz's arguments was ensured protection from Zurg and his forces.
Just how long Zurg has been around threatening the Galactic Alliance is also unknown, but the general consensus is that he is the galaxy's number one enemy.