Green flash

The green flash is a phenomenon that, according to Prunella Deegan, supposedly occurs every year at midnight on New Year's Day. In "Arthur's New Year's Eve," she asks Arthur if he's going to stay up to watch this, but he doesn't understand. She realizes that he's never stayed up till midnight on New Year's Eve before, and therefore doesn't know about the green flash. She explains that every year at midnight, people gather to watch the sky for a most amazing green flash. If it happens, it means that the new year has successfully begun. If it does not happen, then it has to stay the same year for another whole year.

Theories

 * As D.W. Read stays up until midnight and witnesses no green flash, it is possible that the story was entirely a creation of Prunella to try to impress Arthur. Alternatively, it is possible that there really is a green flash, and its absence explains, in part, the the discrepancies in the Arthur timeline.