What is this date format? 2011-08-12T20:17:46.384Z

The T is just a literal to separate the date from the time, and the Z means "zero hour offset" also known as "Zulu time" (UTC). If your strings always have a "Z" you can use:

SimpleDateFormat format = new SimpleDateFormat(
    "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'", Locale.US);
format.setTimeZone(TimeZone.getTimeZone("UTC"));

Or using Joda Time, you can use ISODateTimeFormat.dateTime().

tl;dr

Standard ISO 8601 format is used by your input string.

Instant.parse ( "2011-08-12T20:17:46.384Z" ) 

ISO 8601

This format is defined by the sensible practical standard, ISO 8601.

The T separates the date portion from the time-of-day portion. The Z on the end means UTC (that is, an offset-from-UTC of zero hours-minutes-seconds). The Z is pronounced “Zulu”.

java.time

The old date-time classes bundled with the earliest versions of Java have proven to be poorly designed, confusing, and troublesome. Avoid them.

Instead, use the java.time framework built into Java 8 and later. The java.time classes supplant both the old date-time classes and the highly successful Joda-Time library.

The java.time classes use ISO 8601 by default when parsing/generating textual representations of date-time values.

The Instant class represents a moment on the timeline in UTC with a resolution of nanoseconds. That class can directly parse your input string without bothering to define a formatting pattern.

Instant instant = Instant.parse ( "2011-08-12T20:17:46.384Z" ) ;

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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.

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

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

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. Hibernate 5 & JPA 2.2 support java.time.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, Java SE 10, Java SE 11, and later - Part of the standard Java API with a bundled implementation.
  • Java 9 brought some minor features and fixes.
• Java SE 6 and Java SE 7
  • Most of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android (26+) bundle implementations of the java.time classes.
  • For earlier Android (<26), a process known as API desugaring brings a subset of the java.time functionality not originally built into Android.
    • If the desugaring does not offer what you need, the ThreeTenABP project adapts ThreeTen-Backport (mentioned above) to Android. See How to use ThreeTenABP….

Not sure about the Java parsing, but that's ISO8601: http://en.wikipedia.org/wiki/ISO_8601