Time Regression

 Time Regression (時間退行）is the first phenomenon caused by No.2 and the third phenomenon the members of the StuCS''' experience. This phenomenon was featured during the Kako Random arc.

Description
Random members of the club physically and mentally revert to younger ages between the hours of 12 - 5 pm. As such, while reverted, the members will act in a manner appropriate to their respective ages and do not recall memories that only their older selves would know of. The affected also forget about the phenomenon and do not question their situation, though they do retain a degree of knowledge of their fellow club members. When the members eventually return to their normal ages, their memories from their younger ages will still be clear to them. The exact nature of this phenomenon was not known as it progressed. Inaba hypothesized that it could be that their present selves have switched with their past selves and that anything happening to their past selves may affect the present. An example Inaba gave was if one of then were in a car accident to their present selves when they turn into children, there could be a dead body in the room.

Initially, only Taichi remains completely immune to the phenomenon. However, after Taichi reveals the existence of No.2 to the other members, the 12 - 5 pm time frame and Taichi's immunity were disregarded, allowing the phenomenon to affect all members at random times throughout the entire day, and possibly for the rest of their lives.

Trivia

 * This is the only phenomenon that causes a physical change which is visible to the naked eye.
 * As of Kako Random, Inaba claims this phenomenon scares her the most.
 * The phenomenon was not as serious in the anime as in the novel. The basic necessities such as going to the washroom, finding food, heat, and money were not mentioned in the anime. Taichi even mentioned that they are in a life or death situation if someone were to get sick. Inaba was worried that if someone found out about their age reversions, they might be taken to a research facility and studied.