Short answer
Duration is time-based (seconds and nanos) and Period is date-based (years, months, days).
Explanation
Across a daylight-saving transition they diverge: adding a Period of 1 day to a ZonedDateTime keeps the same wall-clock time the next day (which may be 23 or 25 real hours), while adding a Duration of 24 hours advances exactly 24 hours of elapsed time, landing on a different wall-clock time. Choosing the wrong one produces scheduling bugs that surface only twice a year, which is exactly why interviewers probe it.
Common mistakes
- Saying Duration and Period are interchangeable.
- Forgetting daylight-saving changes make a day not always 24 hours.