CustomCalendarSerializer
KSerializer for CustomCalendar - full struct form. All eleven fields are serialized; defaults from the data class are honored on the way back in so older payloads missing dayOfWeekInMonth / dayOfWeek / dayOfYear / weekOfMonth / weekOfYear still decode (those fields default to -1).
No string form is provided - CustomCalendar is a fully-detailed calendar record, not just a date. Use SimpleDateSerializer if you only need year/month/day.