old_contents: class Timer:
      '''
      This class should spawn another thread that will keep track of a target time
      and compare it to the current system time in order to see how much time is left
      '''
      def __init__(self):
          pass
  
      def start_timer(self, duration):
          '''
          Starts a new timer with the duration which is given in seconds
          '''
          pass
  
      def extend_duration(self, duration):
          '''
          Adds time to the timer based on the duration which is given in seconds
          '''
          pass
  
      def get_time_left(self):
          '''
          Returns the amount of time left in seconds
          Returns -1 if the most recent timer has expired/there is no timer being tracked
          '''
          pass
  
new_contents: class Timer:
      """
      This class should spawn another thread that will keep track of a target time
      and compare it to the current system time in order to see how much time is left
      """
      def __init__(self, timer_type, goal_name=None):
          self.active = False
          self.timer_type = timer_type
          self.goal_name = goal_name
  
      def start_timer(self, duration):
          """Starts a new timer with the duration (seconds) and sets timer to active"""
          pass
  
      def reset(self):
          """Stops the current timer (if any) and sets timer to inactive"""
          pass
  
      def is_running(self):
          """Returns true if the timer is currently running"""
          return self.active
  
subject: Add API for Timer (#421)