How to convert ISO8601 format into milliseconds?

tl;dr

Instant.parse( "2014-10-23T00:35:14.800Z" )
       .toEpochMilli()

One-Liner In java.time

The java.time framework is built into Java 8 and later. These new classes supplant the old date-time classes bundled with the earliest versions of Java such as java.util.Date/.Calendar. See Tutorial. The java.time classes also supplant the highly successful Joda-Time library, being built by some of the same folks including being led by the same Stephen Colbourne.

An Instant is a moment on the timeline in UTC with a resolution of nanoseconds. You can ask it for a count of milliseconds from its epoch (first moment of 1970 in UTC). But remember that an Instant may have additional data, nanoseconds being finer than milliseconds. So you may be losing data in that tiny fraction of a fraction of a second.

The java.time classes use standard ISO 8601 formats when parsing/generating strings. No need to specify a formatting pattern. The Instant class can directly parse a string.

Instant.parse( "2014-10-23T00:35:14.800Z" )

You can convert that to a count of milliseconds since the epoch of first moment of 1970 in UTC by calling toEpochMilli

Be aware of possible data loss as the Instant class can hold nanoseconds. So extracting milliseconds will be truncating any microseconds or nanoseconds in any fractional second. Your example string has only three digits in the fractional second, so that is only milliseconds. But six or nine digits of decimal fraction would be truncated to three when converted to a count of milliseconds.

long millisFromEpoch = Instant.parse( "2014-10-23T00:35:14.800Z" ).toEpochMilli();

To get elapsed time in terms of hours-minutes-seconds, use the Duration class. Feed its between method a pair of moments in time.

Duration duration = Duration.between( Instant.parse( "2014-10-23T00:35:14.800Z" ) , Instant.now() );

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About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, Java SE 10, and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….

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One-Liner In Joda-Time

UPDATE: The Joda-Time project is in maintenance mode, with the team advising migration to the java.time classes.

With the Joda-Time 2.5 library:

long millisSinceEpoch = new DateTime( "2014-10-23T00:35:14.800Z" ).getMillis();

Joda-Time parses and generates ISO 8601 strings by default. Joda-Time works in Android. The java.util.Date/.Calendar classes are notoriously troublesome, confusing, and flawed. Avoid them.

So, turns out the answer was simpler than I would have imagined.

private void setTimestamp(String timeCreated) {
    DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSZ");
    try {
        Date timeCreatedDate = dateFormat.parse(timeCreated);
        timeStamp = (String) DateUtils.getRelativeTimeSpanString(timeCreatedDate.getTime(),
                                  System.currentTimeMillis(), 
                                  DateUtils.SECONDS_IN_MILLIS);
    } catch ( ParseException e) {}
}

That'll work.