我如何获得自纪元以来经过的微秒长（时间），然后在 Java 8 中从同一时间获得月份和年份

Question

I found this code:

long timeStampSeconds = ChronoUnit.MICROS.between(Instant.EPOCH, Instant.now());

but when I converted this back to fetch month and year using the following code, it gives the wrong output.

try {
    epochLong = Long.parseLong(epoch);
} catch (RuntimeException e) {
    //Log exception
    return null;
}

Calendar cal = Calendar.getInstance();
cal.setTimeInMillis(epochLong);
String month = String.valueOf(cal.get(Calendar.MONTH));
String year = String.valueOf(cal.get(Calendar.YEAR));
//print month and year

Any pointers as to how I can generate this in microseconds and get the month and year back right? Please help.

Answer 1

If you are using java.time from Java-8 you can use:

long timeStampSeconds = ChronoUnit.MICROS.between(Instant.EPOCH, Instant.now());

to back you can convert this to an instant, then the instant to the ZoneDateTime

Instant inst = Instant.EPOCH.plus(timeStampSeconds, TimeUnit.MICROSECONDS.toChronoUnit());
ZonedDateTime zdt = inst.atZone(ZoneId.of("Europe/Paris"));

Then you can get the month, year, day, hour, ...

Month month = zdt.getMonth();
int year = zdt.getYear();

Outputs

MICROS:       1574073141066090
ZoneDateTime: 2019-11-18T11:32:21.066090+01:00[Europe/Paris]

Answer 2

It depends on the platform and the Java version.

On my Mac example timeStampSeconds values from your code include

• using jdk-1.8.0_121: 1574193249064000 and 1574193338130000; so always ending in 000
• using jdk-9.0.4: 1574193430428678 and 1574193438362321; so usually not ending in 000.

On Java 8 the now methods of the java.time classes haven't got any finer granularity than millisenods, so the microsecond of millisecond will always be 000. On Java 9 a finer granularity is available if the platform supports it, which may not be the case for all platforms.

So if you're getting 000 always, the solution is to upgrade at least to Java 9 and run your Java program on a computer that supports microsecond precision.

Don't use Calendar

The Calendar class is poorly designed and long outdated. Also it never supports any unit finer than milliseconds. To convert your microsecond value back to a date and time object, use for example:

    long timeStampSeconds = 1_574_193_438_362_321L;
    Instant convertedBack = Instant.EPOCH.plus(timeStampSeconds, ChronoUnit.MICROS);
    System.out.println(convertedBack);

2019-11-19T19:57:18.362321Z

You will notice that the full microsecond precision is preserved. YCF_L in another answer has already shown how to get the year and month values.