Time perception is managed by a complex network involving the basal ganglia and the prefrontal cortex, which together act as the brain's "stopwatch." This circuit requires a steady supply of dopamine to keep accurate time. In the dopamine-deficient ADHD brain, the stopwatch frequently pauses or runs at altered speeds, causing severe distortions in subjective time perception.
Secondly, the ADHD brain suffers from 'working memory' deficits during transitions. 'Getting ready to leave' is not one task; it is 20 micro-tasks (shoes, coat, lock door, find keys, start car). Because the working memory cannot hold the entire sequence simultaneously, the person becomes easily distracted by side-quests. The brain gets hijacked by a random visual stimulus (a piece of mail on the counter) and completely drops the primary objective (leaving the house).
Finally, adrenaline addiction plays a role. The ADHD brain is under-stimulated. Leaving early is boring. The "rush" of leaving at the absolute last possible second creates an intense spike of cortisol and adrenaline. This chemical flood temporarily "fixes" the executive dysfunction, giving you the focus to sprint out the door. The brain subconsciously engineers a last-minute crisis because it is the only way to generate enough fuel to start the engine.
