The calculation of prev execution from the current time does seem to return None when
the current time is 1st of every month and previous month is less than 31 days.

This happens regardless of whether you pass LocalDate or LocalDateTime
ex: If the cron expression is val cron = cron"0 0 0 * * ?" (daily) , then cron.prev(LocalDate.of(2019, 3, 1).atStartOfDay) returns None (Feb has less than 31 days) and cron.prev(LocalDate.of(2019, 4, 1).atStartOfDay) returns 31st of March.