The Period class in java.time handles only the date-oriented potion: years, months, days.

What about the time portion: hours, minutes, seconds?

How can we parse and generate string representations of full periods as defined in ISO 8601, PnYnMnDTnHnMnS? For example, a day and a half: P1DT12H. The academic year is nine months, P9M. Every year I get two weeks and 3 days of vacation, P17D. The customer occupied the hotel room for 2 days and seventeen and a half hours, P2DT17H30M.

The Period class in Joda-Time handles full period. Why not in java.time? Is there some other mechanism?

org.threeten.extra.PeriodDuration

The ThreeTen-Extra project offers a class combining a Period and a Duration. Simply called PeriodDuration.

An amount of time in the ISO-8601 calendar system that combines a period and a duration.

This class models a quantity or amount of time in terms of a Period and Duration. A period is a date-based amount of time, consisting of years, months and days. A duration is a time-based amount of time, consisting of seconds and nanoseconds. See the Period and Duration classes for more details.

The days in a period take account of daylight saving changes (23 or 25 hour days). When performing calculations, the period is added first, then the duration.

Caveat: Be sure to read the Answer by JodaStephen to understand the issues involved in trying to combine Period and Duration. It rarely makes sense to do so in practice, though that is counter to our intuition.

FYI, ThreeTen-Extra, java.time in JSR 310, and Joda-Time are all led by the same man, Stephen Colebourne a.k.a. JodaStephen.