The reckoning of seasons in Unova, with its sharp divides that do not necessarily correlate to either the length of the day or any sharp changes in the weather, is based on the color of the local Deerling's fur. Deerling shed their coats four times a year, replacing them with ones intended to provide more effective camouflage – pink for the flowers of spring, green for the grass of summer, tan for the leaves of autumn, and a dark brown for the broken branches and dead plants of winter. The timing of this shedding is imprecise, for not only does nature never change as abruptly as these pokemon do, but their fur is on a biological cycle exactly three months long (which although not perfect from an evolutionary perspective, is certainly an advantage over the fur of their predecessors, which did not change color at all) while the phases of plant life which it emulates are far more affected by local weather conditions.

For much of Unovan history, most people were farmers who determined the season by local plant life, and the so-called "Deerling calendar" was only used for large national festivals. But in the industrial revolution, regularity and synchronization became far more important for most people than the actual state of local flora, and the approximations represented by the Deerling calendar came into complete regional use. It has failed to spread to the rest of the world, however, where seasons are less extreme and Deerling rare if not unknown.

On the first of every Unovan "month" (a term used loosely, for the Deerling calendar is seasonal and its months take over three lunar cycles) travelers across Unova venture to routes six and seven to see the Deerling's new fur coats and celebrate the coming of new seasons.