function Timer(options) {
    options = options || {};

    const now = options.now || defaultNow;
    let startTime;

    /**
     * Starts the timer.
     * @function
     * @name Timer#start
     */
    this.start = function() {
      startTime = now();
    };

    /**
     * Determines the time since the timer was started.
     * @function
     * @name Timer#elapsed
     * @returns {number} Elapsed time in milliseconds, or NaN if the timer has not been started
     */
    this.elapsed = function() {
      return now() - startTime;
    };
  }
@class Timer
@classdesc Tracks elapsed time
@example
const timer = new jasmine.Timer();
timer.start();
const elapsed = timer.elapsed()