Are leap seconds catered for by Calendar?

Are leap seconds catered for by the GregorianCalendar class?

If not, does any 3rd party library cater for it?

java.time

In modern Java, use only the java.time classes that supplanted the terribly-flawed legacy date-time classes. Never use Calendar, Date, SimpleDateFormat, etc.

Generally best to ignore Leap Second

The java.time framework defines its own time-scale, the Java Time-Scale. For the details, see the explanation in the Javadoc of Instant.

The upshot is java.time ignores leap seconds. The java.time classes by default pretend that every day is exactly 24 hours long. This makes sense as the purpose of Leap Second is to slow down our clocks to keep in synch with our calendar.

• So for conventional consumer and business apps, we want to ignore the Leap Second. Just use the java.time classes bundled with every JDK. Ex: Instant.now() & ZonedDateTime.now(…). And for SQL JDBC database work, OffsetDateTime.
• If you are doing specialized work such as literal rocket science, satellite positioning, etc., then you may want to add the ThreeTen-Extra library to your project. There you can find classes for TAI/UTC times. Be sure to study the Javadoc. (Caveat: I am not sure of the limits of these classes, nor am I comprehend appropriate use-cases.)

See also the Question, Duration with leap seconds.