SerializationFeature has WRITE_DATES_AS_TIMESTAMPS : false
DateTimeFeature.WRITE_DATES_AS_TIMESTAMPS default  : false

defaults (ISO-8601):
  {"day":"2026-09-15","startsAt":"2026-09-15T10:30:00","recordedAt":"2026-09-15T10:30:00Z","zoned":"2026-09-15T10:30:00Z","length":"PT45M"}
with WRITE_DATES_AS_TIMESTAMPS enabled:
  {"day":[2026,9,15],"startsAt":[2026,9,15,10,30],"recordedAt":1789468200.000000000,"zoned":1789468200.000000000,"length":"PT45M"}
numeric form reads back: 2026-09-15