The "City Outside the Universe" is a supernatural location which supposedly exists outside the flow of time, and it is where everything the time anomalies absorb is taken to. Whatever or whoever ends up in the city is erased from the flow time as if they never existed, and is therefore forgotten about. This location made its only appearance in "Last Day of Summer".
The city is a grey and lifeless space, devoid of movement, sound, or life aside from the temporary human and animal residents which were brought there via the time anomalies. Structures that resemble city buildings are the only things present in the space other than the miscellaneous objects that were brought there by the anomalies. According to Phineas Flynn, these mysterious buildings curiously make the space resemble the city of Danville. Phineas, Ferb, Candace, Isabella, Buford, and Baljeet are the only known people to have ever been present in the city; however, other lifeforms which have been brought there include tigers and groundhogs.