How can I get a timestamp for today 9am or tomorrow 9am depending on if we are 9am?

Question

I have a spring boot backend and I want to publish news at 9am UTC+1 everyday.

I would like to get a java.time.Instant for:

• today 9am UTC+1
• tomorrow 9am UTC+1

Depending on if we 9am, how can I do it reliably for all my clients?

Answer 1

You can use OffsetDateTime like this:

LocalTime targetTime = LocalTime.of(9, 0);

OffsetDateTime dateTime = OffsetDateTime.now(ZoneOffset.ofHours(1));
if (dateTime.toLocalTime().compareTo(targetTime) >= 0)
    dateTime = dateTime.plusDays(1);
Instant instant = dateTime.with(targetTime).toInstant();

System.out.println(instant);

Output (executed at 2020-03-17T14:47+01:00)

2020-03-18T08:00:00Z

Change the >= to > if exactly 9 AM should stay as today.

Answer 2

After clarifying some misunderstandings, the answer was adjusted to use the default time zone of the client and return an Instant.

That means, you can use a ZonedDateTime with the system zone for this, have a look at the following method and its comments:

public static Instant determineNextNewsRelease() {
    // get the current time using the default time zone of the client
    ZonedDateTime now = ZonedDateTime.now(ZoneId.systemDefault());
    // get 9 AM using the same day/date
    ZonedDateTime nineAM = now.with(LocalTime.of(9, 0));

    // and check if now is before nineAM
    if (now.isBefore(nineAM)) {
        return nineAM.toInstant();
    } else {
        return nineAM.plusDays(1).toInstant();
    }
}

Use the method like this:

public static void main(String[] args) {
    Instant nextNewsRelease = determineNextNewsRelease();
    // and print the result
    System.out.println("Next news will be released at "
            + nextNewsRelease.toEpochMilli() + " (" 
            + ZonedDateTime.ofInstant(nextNewsRelease, ZoneId.systemDefault())
            + ")");
}

The result printed is

Next news will be released at 1584518400000 (2020-03-18T09:00+01:00[Europe/Berlin])

Try it...