如何在Java中获取自定义日期和时间的纪元？

Question

Calendar now = Calendar.getInstance();
        int year = now.get(Calendar.YEAR);
        int month=now.get(Calendar.MONTH)+1;
        int date=now.get(Calendar.DATE);
        String yearInString = String.valueOf(year);
        String monthInString=String.valueOf(month);
        if(monthInString.length()==1){
            monthInString="0"+monthInString;
        }
        String dateInString=String.valueOf(date);
        if(dateInString.length()==1){
            dateInString="0"+dateInString;
        }
        int hour=now.get(Calendar.HOUR);
        String hourInString=String.valueOf(hour);
        int minute=now.get(Calendar.MINUTE);
        String minuteInString=String.valueOf(minute);
        if(minuteInString.length()==1){
            minuteInString="0"+minuteInString;
        }
        int second=now.get(Calendar.SECOND);
        String secondInString=String.valueOf(second);
        String HRD=yearInString+"-"+monthInString+"-"+dateInString+" "+hourInString+":"+minuteInString+":"+secondInString;
        System.out.println(HRD);
        DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-dd-MM HH:mm:ss.SSS");
        LocalDateTime dt = LocalDateTime.parse(HRD, dtf);
        Instant instant = dt.toInstant(ZoneOffset.UTC);
        System.out.println(instant.toEpochMilli());

Date date1 = dateFormat.parse(HRD);
long epoch = date1.getTime();
System.out.println(epoch);

但显示错误

线程“main”java.time.format.dateTimeParseException:Text'2021-10-06 0:29:43“无法在索引11处解析，位于java.base/java.time.format.dateTimeFormatter.parseresolved0(DateTimeFormatter.java:2052)，java.base/java.time.format.dateTimeFormatter.parse（DateTimeFormatter.java:1954)，java.base/java.time.localDateTime.parse（

Answer 1

java.utilDate-Time API已经过时并且容易出错。建议完全停止使用它，并切换到现代日期时间API^*。

使用java.time的解决方案，现代的日期-时间API：从下午12点的offsetdatetime中，您可以使用offsetdatetime#toinstant获得相应的instant，并且从这个instant，您可以获得纪元毫秒。

演示：

import java.time.LocalDate;
import java.time.LocalTime;
import java.time.OffsetDateTime;
import java.time.ZoneOffset;
import java.time.temporal.TemporalAdjusters;

public class Main {
    public static void main(String[] args) {
        OffsetDateTime todayAtNoon = OffsetDateTime.of(LocalDate.now(ZoneOffset.UTC), LocalTime.NOON, ZoneOffset.UTC);
        OffsetDateTime lastDateOfMonth = todayAtNoon.with(TemporalAdjusters.lastDayOfMonth());
        for (OffsetDateTime odt = todayAtNoon; !odt.isAfter(lastDateOfMonth); odt = odt.plusDays(1)) {
            System.out.println(odt.toInstant().toEpochMilli());
        }
    }
}

1633521600000
1633608000000
1633694400000
...