Timer functionality

Adds the ability to set a timer that counts down from set time. Timer does not need to be system time aware and can demonstrate the behavior of current time functionality by using a similar implementation.

Start by locating the existing current-time functionality and trace how its behavior is implemented. Define the timer's expected countdown behavior from a set duration, confirm it does not depend on system time, and use the existing functionality's implementation as the comparison point for completion.

Written by the indexing model from the issue text.