[rabestro]

pull request

Description

This PR adds a new concept, durations-and-periods, and a concept exercise that teaches it, gym-front-desk.

The idea was discussed and approved on the forum: Add a concept for Duration and Period.

Motivation

The datetime concept covers points in time (LocalDate, LocalDateTime) but not amounts of time, and beauty-salon-regulars explicitly leaves Duration and Period out of scope.
Students who do date arithmetic in practice exercises either do it by hand or run into the usual pitfalls, the most common being Period.between(a, b).getDays() used as a total number of days.

Concept: durations-and-periods

• introduction.md: Period (factory methods, plus(), month-end clamping, Period.between() and its parts), counting units with ChronoUnit.X.between(), and Duration (factory methods, Duration.between(), toXxx() vs toXxxPart()).
• about.md: adds ISO-8601 parsing, until(), toTotalMonths(), normalized() and unit-by-unit equals(), UnsupportedTemporalTypeException when mixing dates and durations, comparing and arithmetic with durations, and a short aside on why Period.ofDays(1) and Duration.ofDays(1) differ across a DST change.
• Javadoc links point to Java 25, each sentence is on its own line, and the docs do not refer to any practice exercise.
• All code examples were checked in jshell.

Exercise: gym-front-desk

A climbing gym front desk, with five tasks:

1. membershipEnd(): add a Period of months to a start date (tests include month-end clamping and leap years).
2. membershipLength(): Period.between().
3. daysLeft(): total days with ChronoUnit.DAYS.between() (a test spans several months, so Period.getDays() fails it).
4. visitDuration(): Duration.between() for a visit, including one past midnight.
5. formatVisit(): "1h 35m" on a receipt, using toHours() and toMinutesPart() (a test with a visit over 24 hours catches toHoursPart()).

Prerequisites: datetime, strings.
design.md lists suggested analyzer rules.

Config changes

• Added the concept and the exercise to config.json (UUIDs 16500ebe-104e-44c3-84b3-3126a49bdb89 and 6553f255-154f-441d-a6e1-a428434f9653) and the exercise to exercises/settings.gradle.
• Added durations-and-periods to the practices and prerequisites of gigasecond, which adds a gigasecond to a date-time.
  Happy to drop it from prerequisites if you'd rather not lock the exercise behind the new concept.

Checks run locally

• ./gradlew test, check and compileStarterTestJava for the exercise (JDK 25): 23 tests pass, no Checkstyle issues.
• configlet lint and configlet generate (the exercise's introduction.md is generated from the concept).
• markdownlint-cli2 on the new Markdown files.

The i18n / completeness check will need the ready-to-translate label to queue the new English for translation.

---

Reviewer Resources:

Track Policies

🤖 Generated with Claude Code