可以先转为Timestamp或时间戳

具体如下

    public static void main(String[] args) {
        LocalDateTime now = LocalDateTime.now();

        System.out.println(System.currentTimeMillis());  //当前系统时刻（亚洲/上海）1670130498855
//LocalDateTime 转时间毫秒值
         //方式一
        long time = Timestamp.valueOf(now).getTime();
        System.out.println(time);                        //与+8是和当前系统时刻相等（亚洲/上海）1670130498855

        //方式二
        long time8 = now.toInstant(ZoneOffset.of("+8")).toEpochMilli();
        System.out.println(time8);                       //与当前系统时刻相等（亚洲/上海）1670130498855

        
        //其他注意
        LocalDateTime now1 = LocalDateTime.now(ZoneOffset.UTC);
        long time0 = now1.toInstant(ZoneOffset.of("+0")).toEpochMilli();
        long timeUtc = now1.toInstant(ZoneOffset.UTC).toEpochMilli();
        System.out.println(time0);                                       //1670130498855
        System.out.println(timeUtc);                                     //1670130498855


    }