How to send a HTTP request at a very precise time in JAVA

I want to send several, parallel HTTP requests (for example via async OkHttp) in a very precise time window (eg from 20.08.2020 00.00.00 to 20.08.2020 00.00.50) .

http://www.quartz-scheduler.org has a precision up to 1second.

How to schedule them?

If you care about the time they are received by the server as opposed to when the client starts sending then you should prewarm OkHttp with some requests.

For HTTP/1.1 connections you need multiple connection. Check the connection pool size and tune it if needed.

For a HTTP/2 connection ready when you go to send results. If you are worried about the size of any one request, you may want to override the default behaviour of OkHttp by having multiple client instances to avoid head of line blocking on the shared socket.

As suggested above, for java thread scheduling use ScheduledExecutorService and probably wake before the event and spin until the exact millisecond. You can't use nanoTime since it's related to an arbitrary epoch, so millisecond accuracy is probably the best you can do.

You can use Scheduling a task in Java within a CompletableFuture to schedule your task:
Something like this to scheule your http task:

TimeUnit can be used to schedule till milliseconds.(TimeUnit.MILLISECONDS)

    public static <T> CompletableFuture<T> schedule(
    ScheduledExecutorService executor,
    Supplier<T> command,
    long delay,
    TimeUnit unit
) {
    CompletableFuture<T> completableFuture = new CompletableFuture<>();
    executor.schedule(
        (() -> {
            try {
                return completableFuture.complete(command.get());
            } catch (Throwable t) {
                return completableFuture.completeExceptionally(t);
            }
        }),
        delay,
        unit
    );
    return completableFuture;
}

Refer this article for concept for completablefuture:

https://www.artificialworlds.net/blog/2019/04/05/scheduling-a-task-in-java-within-a-completablefuture/

Alternate you can write your own scheduler:

https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ScheduledExecutorService.html

EDIT:

Try using this :
scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit)

ZonedDateTime now = ZonedDateTime.now(ZoneId.of("America/Los_Angeles"));

 
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSS");
LocalDateTime ldt= LocalDateTime.parse("2020-10-17T12:42:04.000", formatter);
ZonedDateTime nextRun= ldt.atZone(ZoneId.of("America/Los_Angeles"));

        


if(now.compareTo(nextRun) > 0)
    nextRun = nextRun.plusDays(1);

Duration duration = Duration.between(now, nextRun);
long initalDelay = duration.toMillis();


ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);            
scheduler.scheduleAtFixedRate(new MyRunnableTask(),
    initalDelay,
    TimeUnit.DAYS.toMillis(1),
    TimeUnit.MILLISECONDS);