One of the Monogatari series’ greatest strengths is its inadherence to chronology. It often eschews placing events in chronological order to focus on a particular emotional narrative or relationship. The anime adaptation plays with this visually, revealing tidbits in background details that further inform viewers upon rewatching the series as a whole.
Koyomimonogatari is a series of short, seemingly frivolous episodes tertiary to the main storyline. They’re short diversions that span the length of what Monogatari arcs have aired, plopping the viewer into the center of that specific timeframe before jumping ahead to the middle of the next narrative arc. Chronology is usually discarded by the Monogatari series, but it has a deliberate role in Koyomimonogatari.