LocalDateTime 获取当前（年/月）第一天及最后一天 及 获取当天起始时间

// 获取今年第一天及最后一天
LocalDateTime firstDayOfYear = LocalDateTime.of(LocalDate.from(LocalDateTime.now().with(TemporalAdjusters.firstDayOfYear())), LocalTime.MIN);
LocalDateTime lastDayOfYear = LocalDateTime.of(LocalDate.from(LocalDateTime.now().with(TemporalAdjusters.lastDayOfYear())), LocalTime.MAX);

// 获取当前月第一天及最后一天
LocalDateTime firstDayOfMonth = LocalDateTime.of(LocalDate.from(LocalDateTime.now().with(TemporalAdjusters.firstDayOfMonth())), LocalTime.MIN);
LocalDateTime lastDayOfMonth = LocalDateTime.of(LocalDate.from(LocalDateTime.now().with(TemporalAdjusters.lastDayOfMonth())), LocalTime.MAX);


// 获取当天的起始时间
LocalDateTime firstTimeOfDay = LocalDateTime.of(LocalDate.now(), LocalTime.MIN);
LocalDateTime lastTimeOfDay = LocalDateTime.of(LocalDate.now(), LocalTime.MAX);