为什么不同日期的时间戳没有部分时间是相同的？

Question

    public static void main(String[] args) throws ParseException {
        long DAY = 1000 * 60 * 60 * 24;

        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        Date yDate = simpleDateFormat.parse("2020-06-12 23:58:33");
        System.out.println(yDate.getTime() - (yDate.getTime() % DAY));

        Date date = simpleDateFormat.parse("2020-06-13 00:29:38");
        System.out.println(date.getTime() - date.getTime() % DAY);
    }

控制台打印：1591920000000 1591920000000

Answer 1

这是一个解决您的问题的示例，java.time而不是使用过时的类java.util：

public static void main(String[] args) {
    // define an up-to-date(-time) formatter and specify the output language
    DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss",
                                                    Locale.ENGLISH);
    // define two timezones to be used later
    ZoneId shanghai = ZoneId.of("Asia/Shanghai");
    ZoneId utc = ZoneId.of("UTC");

    /*
     * Parse the datetimes to a ZonedDateTime each, because timezones matter here...
     * This uses two different ones in order to highlight how timezones matter here
     */
    ZonedDateTime zdtShanghaiOne = LocalDateTime.parse("2020-06-12 23:58:33", dtf)
                                                    .atZone(shanghai);
    ZonedDateTime zdtShanghaiTwo = LocalDateTime.parse("2020-06-13 00:29:38", dtf)
                                                    .atZone(shanghai);
    ZonedDateTime zdtUtcOne = LocalDateTime.parse("2020-06-12 23:58:33", dtf)
                                                    .atZone(utc);
    ZonedDateTime zdtUtcTwo = LocalDateTime.parse("2020-06-13 00:29:38", dtf)
                                                    .atZone(utc);
    /*
     * Print the millis as a result of subtracting the time information.
     * Acutally, this way to get a datetime of the form "yyyy-MM-dd'T'00:00:00.0'Zone'"
     * takes the datetime and returns the same date with a time of 0 hours, minutes, seconds...
     */
    System.out.println("Shanghai millis day one:\t"
                + zdtShanghaiOne.with(LocalTime.MIN).toInstant().toEpochMilli());
    System.out.println("Shanghai millis day two:\t"
                + zdtShanghaiTwo.with(LocalTime.MIN).toInstant().toEpochMilli());
    System.out.println("UTC millis day one:\t\t"
                + zdtUtcOne.with(LocalTime.MIN).toInstant().toEpochMilli());
    System.out.println("UTC millis day two:\t\t"
                + zdtUtcTwo.with(LocalTime.MIN).toInstant().toEpochMilli());
}

这导致输出

public static void main(String[] args) {
    // define an up-to-date(-time) formatter and specify the output language
    DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss",
                                                    Locale.ENGLISH);
    // define two timezones to be used later
    ZoneId shanghai = ZoneId.of("Asia/Shanghai");
    ZoneId utc = ZoneId.of("UTC");

    /*
     * Parse the datetimes to a ZonedDateTime each, because timezones matter here...
     * This uses two different ones in order to highlight how timezones matter here
     */
    ZonedDateTime zdtShanghaiOne = LocalDateTime.parse("2020-06-12 23:58:33", dtf)
                                                    .atZone(shanghai);
    ZonedDateTime zdtShanghaiTwo = LocalDateTime.parse("2020-06-13 00:29:38", dtf)
                                                    .atZone(shanghai);
    ZonedDateTime zdtUtcOne = LocalDateTime.parse("2020-06-12 23:58:33", dtf)
                                                    .atZone(utc);
    ZonedDateTime zdtUtcTwo = LocalDateTime.parse("2020-06-13 00:29:38", dtf)
                                                    .atZone(utc);
    /*
     * Print the millis as a result of subtracting the time information.
     * Acutally, this way to get a datetime of the form "yyyy-MM-dd'T'00:00:00.0'Zone'"
     * takes the datetime and returns the same date with a time of 0 hours, minutes, seconds...
     */
    System.out.println("Shanghai millis day one:\t"
                + zdtShanghaiOne.with(LocalTime.MIN).toInstant().toEpochMilli());
    System.out.println("Shanghai millis day two:\t"
                + zdtShanghaiTwo.with(LocalTime.MIN).toInstant().toEpochMilli());
    System.out.println("UTC millis day one:\t\t"
                + zdtUtcOne.with(LocalTime.MIN).toInstant().toEpochMilli());
    System.out.println("UTC millis day two:\t\t"
                + zdtUtcTwo.with(LocalTime.MIN).toInstant().toEpochMilli());
}

真的建议使用java.time不必依赖自己的计算，这些计算可能会因缺少时区信息（编码和/或编码器）而失败。