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1. Overview

In this quick tutorial, we’ll illustrate multiple ways of converting time into Unix-epoch milliseconds in Java.

More specifically, we’ll use:

  • Core Java’s java.util.Date and Calendar
  • Java 8’s Date and Time API
  • Joda-Time library

2. Core Java

2.1. Using Date

Firstly, let’s define a millis property holding a random value of milliseconds:

long millis = 1556175797428L; // April 25, 2019 7:03:17.428 UTC

We’ll use this value to initialize our various objects and verify our results.

Next, let’s start with a Date object:

Date date = // implementation details

Now, we’re ready to convert the date into milliseconds by simply invoking the getTime() method:

Assert.assertEquals(millis, date.getTime());

2.2. Using Calendar

Likewise, if we have a Calendar object, we can use the getTimeInMillis() method:

Calendar calendar = // implementation details
Assert.assertEquals(millis, calendar.getTimeInMillis());

3. Java 8 Date Time API

3.1. Using Instant

Simply put, Instant is a point in Java’s epoch timeline.

We can get the current time in milliseconds from the Instant:

java.time.Instant instant = // implementation details
Assert.assertEquals(millis, instant.toEpochMilli());

As a result, the toEpochMilli() method returns the same number of milliseconds as we defined earlier.

3.2. Using LocalDateTime

Similarly, we can use Java 8’s Date and Time API to convert a LocalDateTime into milliseconds:

LocalDateTime localDateTime = // implementation details
ZonedDateTime zdt = ZonedDateTime.of(localDateTime, ZoneId.systemDefault());
Assert.assertEquals(millis, zdt.toInstant().toEpochMilli());

First, we created an instance of the current date. After that, we used the toEpochMilli() method to convert the ZonedDateTime into milliseconds.

As we know, LocalDateTime doesn’t contain information about the time zone. In other words, we can’t get milliseconds directly from LocalDateTime instance.

4. Joda-Time

While Java 8 adds much of Joda-Time’s functionality, we may want to use this option if we are on Java 7 or earlier.

4.1. Using Instant

Firstly, we can obtain the current system milliseconds from the Joda-Time Instant class instance using the getMillis() method:

Instant jodaInstant = // implementation details
Assert.assertEquals(millis, jodaInstant.getMillis());

4.2. Using DateTime

Additionally, if we have a Joda-Time DateTime instance:

DateTime jodaDateTime = // implementation details

Then we can retrieve the milliseconds with the getMillis() method:

Assert.assertEquals(millis, jodaDateTime.getMillis());

5. Conclusion

In conclusion, this article demonstrates how to convert time into milliseconds in Java.

Finally, as always, the complete code for this article is available over on GitHub.

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