Java中日期之Date

1.获取两个Date之间的相差多少？（秒、分、时、天）

  try{
            SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

            smdate=sdf.parse(sdf.format(smdate));

            bdate=sdf.parse(sdf.format(bdate));

            Calendar cal = Calendar.getInstance();

            cal.setTime(smdate);

            long time1 = cal.getTimeInMillis();

            cal.setTime(bdate);

            long time2 = cal.getTimeInMillis();

            long between_minutes=(time2-time1)/(1000*60);

           Integer.parseInt(String.valueOf(between_minutes));
        }catch (ParseException e){
            e.printStackTrace();
        }

2.获取一天中的最开始时间

try {
            SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            current = sdf.parse(sdf.format(current));
            Calendar calendar = Calendar.getInstance();
            calendar.setTime(current);
            calendar.set(Calendar.HOUR_OF_DAY,0);
            calendar.set(Calendar.MINUTE,0);
            calendar.set(Calendar.SECOND,0);
            calendar.set(Calendar.MILLISECOND,0);
            Date dayStart = calendar.getTime();
            return dayStart;
        } catch (ParseException e) {
            e.printStackTrace();
        }

3.获取一天中的最结束时间

try {
            SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            current = sdf.parse(sdf.format(current));
            Calendar calendar = Calendar.getInstance();
            calendar.set(Calendar.HOUR_OF_DAY,23);
            calendar.set(Calendar.MINUTE,59);
            calendar.set(Calendar.SECOND,59);
            calendar.set(Calendar.MILLISECOND,999);
            Date dayEnd = calendar.getTime();
            return dayEnd;
        } catch (ParseException e) {
            e.printStackTrace();
        }

4. LocalDateTime 转Date

获取本月开始的时间（LocalDateTime）并转换成Date形式：

public static LocalDateTime getFirstDayOfMonth() {
        LocalDateTime date = LocalDateTime.now();
        LocalDateTime firstday = date.with(TemporalAdjusters.firstDayOfMonth());
        return  LocalDateTime.of(firstday.toLocalDate(), LocalTime.MIN);
    }

 public static Date getDateByFirstDayOfMonth() {
        LocalDateTime minFirestDay =  getFirstDayOfMonth();
        ZoneId zoneId = ZoneId.systemDefault();
        ZonedDateTime zdt = minFirestDay.atZone(zoneId);
        return  Date.from(zdt.toInstant());
    }

 5.

DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd");
LocalDate.now().format(dtf)

 6.LocalDate的常见用法：获取当前日期、获取指定日期、指定日期月最后一天、指定日期月有多少天：

7.LocalDateTime指定日期格式，以及日期的相减：

 DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        String s = LocalDateTime.now().minusDays(1).format(dtf);

当前日期：2020-07-20 14:33:18

上述结果输出为:2020-07-19 14:28:18