Doomsday Trainer

This Android app helps you train the Doomsday Algorithm which allows you to quickly deduce the day of the week to any given date within seconds.

It was invented by John Conway in 1973. Here, a variant is used which was found by Fong and Walters in 2010.

For more information and an actual explanation please see the Wikipedia article. Even though the app contains an explanation, it assumes you are already familiar with it.