Java - custom task scheduler is very innacurate

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Java

scheduler

time

I want to build a very simple, yet efficient, scheduler to which classes that implement a SchedulerListener interface (for started/finished events) register with how many seconds and/or milliseconds must the scheduler wait to notify them (via the finished event).

When a request is finished, the individual object is notified and it calculates the difference between start and finishing times which are then compared to a previously registered "Expected time" so as to know the difference.

Usually differences vary between 0 to 20 milliseconds.

The issue is when differences are showed as negative - meaning the task finished before the time it was expected to have passed.

The scheduler runs on a separate Thread.

Does anything seem to be wrong with the code?

Note: originally the requests were stored in an ArrayList and the delays would go up to 30ms sometimes, but nothing bad at all. As an attempt to bring that value down i replaced the ArrayList with a ConcurrentHashMap (to avoid exceptions while iterating), which does bring down the delay though also causes the stated issue.

What I have tried:

The logic behind the scheduler is easy. Im using a Delta Time mechanic, decrementing each request (in milliseconds) by each "frame" 's deltaTime and unregistering the request after the respective time (or more) has passed.

I won't be posting all of the code to avoid overcluttering the question unless it's needed.

The Scheduler's code

public void registerListener(SchedulerListener listener, long seconds, long milliseconds)
{
    if(!isRunning)
        return;

    listener.onSchedulerStart();
    listeners.put(listener, secondsToMilliseconds(seconds) + milliseconds);
}

public void unregisterListener(SchedulerListener listener)
{
    if(!isRunning)
        return;

    listeners.remove(listener);

public void run()
{
    isRunning = true;

    long lastTime, currentTime, deltaTime;
    lastTime = System.currentTimeMillis();

    while(!Thread.currentThread().isInterrupted())
    {

        currentTime = System.currentTimeMillis();

        deltaTime = currentTime - lastTime;

        checkRequests(deltaTime);

        lastTime = currentTime;
    }
}

private void checkRequests(long deltaTime)
{
    long value;

    for(SchedulerListener listener : listeners.keySet())
    {
        value = listeners.get(listener) - deltaTime;

        listeners.put(listener, value);

        if(value <= 0)
        {
            listener.onSchedulerDone();
            listeners.remove(listener);
        }
    }
}