[java] How to get milliseconds from LocalDateTime in Java 8

I am wondering if there is a way to get current milliseconds since 1-1-1970 (epoch) using the new LocalDate, LocalTime or LocalDateTime classes of Java 8.

The known way is below:

long currentMilliseconds = new Date().getTime();

or

long currentMilliseconds = System.currentTimeMillis();

You can try this:

long diff = LocalDateTime.now().atZone(ZoneOffset.UTC).toInstant().toEpochMilli();

Date and time as String to Long (millis):

String dateTimeString = "2020-12-12T14:34:18.000Z";

DateTimeFormatter formatter = DateTimeFormatter
                .ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'", Locale.ENGLISH);

LocalDateTime localDateTime = LocalDateTime
        .parse(dateTimeString, formatter);

Long dateTimeMillis = localDateTime
        .atZone(ZoneId.systemDefault())
        .toInstant()
        .toEpochMilli();

If you have a Java 8 Clock, then you can use clock.millis() (although it recommends you use clock.instant() to get a Java 8 Instant, as it's more accurate).

Why would you use a Java 8 clock? So in your DI framework you can create a Clock bean:

@Bean
public Clock getClock() {
    return Clock.systemUTC();
}

and then in your tests you can easily Mock it:

@MockBean private Clock clock;

or you can have a different bean:

@Bean
public Clock getClock() {
    return Clock.fixed(instant, zone);
}

which helps with tests that assert dates and times immeasurably.

To get the current time in milliseconds (since the epoch), use System.currentTimeMillis().

Why didn't anyone mentioned the method LocalDateTime.toEpochSecond():

LocalDateTime localDateTime = ... // whatever e.g. LocalDateTime.now()
long time2epoch = localDateTime.toEpochSecond(ZoneOffset.UTC);

This seems way shorter that many suggested answers above...

You can use java.sql.Timestamp also to get milliseconds.

LocalDateTime now = LocalDateTime.now();
long milliSeconds = Timestamp.valueOf(now).getTime();
System.out.println("MilliSeconds: "+milliSeconds);

Since Java 8 you can call java.time.Instant.toEpochMilli().

For example the call

final long currentTimeJava8 = Instant.now().toEpochMilli();

gives you the same results as

final long currentTimeJava1 = System.currentTimeMillis();

What I do so I don't specify a time zone is,

System.out.println("ldt " + LocalDateTime.now().atZone(ZoneId.systemDefault()).toInstant().toEpochMilli());
System.out.println("ctm " + System.currentTimeMillis());

gives

ldt 1424812121078 
ctm 1424812121281

As you can see the numbers are the same except for a small execution time.

Just in case you don't like System.currentTimeMillis, use Instant.now().toEpochMilli()

To avoid ZoneId you can do:

LocalDateTime date = LocalDateTime.of(1970, 1, 1, 0, 0);

System.out.println("Initial Epoch (TimeInMillis): " + date.toInstant(ZoneOffset.ofTotalSeconds(0)).toEpochMilli());

Getting 0 as value, that's right!

  default LocalDateTime getDateFromLong(long timestamp) {
    try {
        return LocalDateTime.ofInstant(Instant.ofEpochMilli(timestamp), ZoneOffset.UTC);
    } catch (DateTimeException tdException) {
      //  throw new 
    }
}

default Long getLongFromDateTime(LocalDateTime dateTime) {
    return dateTime.atOffset(ZoneOffset.UTC).toInstant().toEpochMilli();
}