How to get ISO format from time in milliseconds in Java?

If you have epoch millis. The below line should work,

Instant.ofEpochMilli(millis).toString()

As said by Sridhar Sg's the code:

Instant.ofEpochMilli(millis).toString()

will work as the toString() method will give you the ISO-8601 extended format representation (with separators).

Note that the Instant class will only work from JDK 8 (introduction of the java.time package) unless you use ThreeTen Backport, the backport to Java 6 and 7.

If millis = 1603101879234 the above method will return: 2020-10-19T10:04:39.234Z

If you need other kind of format, like the ISO-8601 basic format (without separators except the T in the middle) you can use a custom DateTimeFormatter like this:

Instant instant = Instant.ofEpochMilli(millis);
DateTimeFormatter outFormatter = DateTimeFormatter
        .ofPattern("yyyyMMdd'T'HHmmss.SSSX") // millisecond precision
        .withZone(ZoneId.of("UTC"));

String basicIso = outFormatter.format(instant);

For the same millis = 1603101879234 the above method will yield: 20201019T100439.234Z.

The question does not mention time zone, so I'll assume you meant UTC/GMT. The question does not explain "ISO format", so I'll assume you meant ISO 8601. This happens to be the default for third-party Joda-Time 2.3 library. This is thread-safe.

// © 2013 Basil Bourque. This source code may be used freely forever by anyone taking full responsibility for doing so.

System.out.println( "That moment: " + new org.joda.time.DateTime( 1344855183166L, org.joda.time.DateTimeZone.UTC ) );

When run…

That moment: 2012-08-13T10:53:03.166Z

If the original poster meant a Poland time zone…

// © 2013 Basil Bourque. This source code may be used freely forever by anyone taking full responsibility for doing so.

// Time Zone list… http://joda-time.sourceforge.net/timezones.html
org.joda.time.DateTimeZone warsawTimeZone = org.joda.time.DateTimeZone.forID( "Europe/Warsaw" );
System.out.println( "That moment in Poland: " + new org.joda.time.DateTime( 1344855183166L, warsawTimeZone ) );

When run…

That moment in Poland: 2012-08-13T12:53:03.166+02:00