The current Zimmerman method considers the combination of temperature, humidity, and precipitation to determine the watering percentage. In a way it does take into account the time of the year, because during the summer the average temperature is higher whereas during the fall and spring the average temperature is lower.
It’s pretty easy to introduce a feature such as ‘monthly watering percentage’, but my worry is that it’s a manual setting and will introduce some confusions to users, whereas the Zimmerman method is supposed to be more automatic.