5 years later I noticed this because I now need 2 calendar apps.
One for work, one for personal, so I googled my annoyance and found this thread.
I'm going to make an educated guess since I work in the mobile industry and have an iPhone, that date, time and location capacities are hard coded into my device OS.
For any Android device (not owned by Google) the calendar app is a service, in order to display the proper date (wherever I am in the worrld) it would constantly have location services allowed, and ping towers constantly and have locatio settings on at all times, requiring an enourmous amount of battery life and data usage creating a much heavier and cumbersone app than intended, all for a dynamic display icon.
Thoughts?