How to get milliseconds from LocalDateTime in Java 8

342
votes

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();
12
javadatetimejava-8millisecondsjava-time
What's wrong with System.currentTimeMillis()? – Dawood ibn Kareem
@DavidWallace He's trying to get the time of a Date, not the current time? – Anubian Noob
"... a way to get current milliseconds ..." – Dawood ibn Kareem
milliseconds counting from 1-1-1970. I was wandering if there is a method to get them using the new classes of Java 8 LocalDate, LocalTime and LocalDateTime, cause i didn't found one. – George Siggouroglou
My understanding of the purpose for those classes is that it is a "human-focused" understanding of time, and currentTimeMillis would be irrelevant in that context. Think Calendar + Wall-clock with really good precision, and no concerns about time zones and locality. So there's no way to get back to "UTC Time" from a LocalTime – Gus

12 Answers

384
votes

I'm not entirely sure what you mean by "current milliseconds" but I'll assume it's the number of milliseconds since the "epoch," namely midnight, January 1, 1970 UTC.

If you want to find the number of milliseconds since the epoch right now, then use System.currentTimeMillis() as Anubian Noob has pointed out. If so, there's no reason to use any of the new java.time APIs to do this.

However, maybe you already have a LocalDateTime or similar object from somewhere and you want to convert it to milliseconds since the epoch. It's not possible to do that directly, since the LocalDateTime family of objects has no notion of what time zone they're in. Thus time zone information needs to be supplied to find the time relative to the epoch, which is in UTC.

Suppose you have a LocalDateTime like this:

LocalDateTime ldt = LocalDateTime.of(2014, 5, 29, 18, 41, 16);

You need to apply the time zone information, giving a ZonedDateTime. I'm in the same time zone as Los Angeles, so I'd do something like this:

ZonedDateTime zdt = ldt.atZone(ZoneId.of("America/Los_Angeles"));

Of course, this makes assumptions about the time zone. And there are edge cases that can occur, for example, if the local time happens to name a time near the Daylight Saving Time (Summer Time) transition. Let's set these aside, but you should be aware that these cases exist.

Anyway, if you can get a valid ZonedDateTime, you can convert this to the number of milliseconds since the epoch, like so:

long millis = zdt.toInstant().toEpochMilli();
105
votes

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()

23
votes

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!

20
votes

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();
16
votes

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);
11
votes

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

7
votes

You can try this:

long diff = LocalDateTime.now().atZone(ZoneOffset.UTC).toInstant().toEpochMilli();
3
votes

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.

3
votes

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...

1
votes
  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();
}
0
votes

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();
0
votes

For LocalDateTime I do it this way:

LocalDateTime.of(2021,3,18,7,17,24,341000000)
    .toInstant(OffsetDateTime.now().getOffset())
    .toEpochMilli()