Timer.java

public class Timer {

    private long startTime;
    private long duration;

    /**
     * Constructs a new timer that records the start time of when the timer was created.
     *
     * @param duration the duration in milliseconds until the timer is expired
     */
    public Timer(long duration) {
        this.duration = duration;
        startTime = System.currentTimeMillis();
    }

    /**
     * Returns if the current time has surpassed the set duration from the start time.
     *
     * @return true if surpassed, false otherwise
     */
    public boolean isRunning() {
        return System.currentTimeMillis() - startTime < duration;
    }

    /**
     * Returns the amount of time remaining in milliseconds before time expires.
     *
     * @return remaining time
     */
    public long remainingTime() {
        return isRunning() ? duration - (System.currentTimeMillis() - startTime) : 0;
    }

}