Get the number of days, weeks, and months, since E

问题:

I'm trying to get the number of days, weeks, months since Epoch in Java.

The Java Calendar class offers things like calendar.get(GregorianCalendar.DAY_OF_YEAR), or Calendar.get(GregorianCalendar.WEEK_OF_YEAR), which is a good start but it doesn't do exactly what I need.

Is there an elegant way to do this in Java?

回答1:

You can use the Joda Time library to do this pretty easily - I use it for anything time related other than using the standard Java Date and Calendar classes. Take a look at the example below using the library:

MutableDateTime epoch = new MutableDateTime();
epoch.setDate(0); //Set to Epoch time
DateTime now = new DateTime();

Days days = Days.daysBetween(epoch, now);
Weeks weeks = Weeks.weeksBetween(epoch, now);
Months months = Months.monthsBetween(epoch, now);

System.out.println("Days Since Epoch: " + days.getDays());
System.out.println("Weeks Since Epoch: " + weeks.getWeeks());
System.out.println("Months Since Epoch: " + months.getMonths());

When I run this I get the following output:

Days Since Epoch: 15122
Weeks Since Epoch: 2160
Months Since Epoch: 496

回答2:

java.time

Use the java.time classes built into Java 8 and later.

LocalDate now = LocalDate.now();
LocalDate epoch = LocalDate.ofEpochDay(0);

System.out.println("Days: " + ChronoUnit.DAYS.between(epoch, now));
System.out.println("Weeks: " + ChronoUnit.WEEKS.between(epoch, now));
System.out.println("Months: " + ChronoUnit.MONTHS.between(epoch, now));

Output

Days: 16857
Weeks: 2408
Months: 553

回答3:

Long currentMilli = System.currentTimeMillis();
Long seconds = currentMilli / 1000;
Long minutes = seconds / 60;
Long hours = minutes / 60;
Long days = hours / 24;
System.out.println("Days since epoch : "  + days);

or

System.out.println("Days since epoch : "  + ((int) currentMilli / 86400000));

回答4:

I'm kind of surprised that almost all answers are actually calculating days between epoch and now. With java.time.LocalDate it's as simple as:

LocalDate.now().toEpochDay()

回答5:

Date.getTime() - Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT represented by this Date object.

You can use this and knowledge of how many milliseconds are in the intervals you care about to do the calculations.

回答6:

Calendar now = Calendar.getInstance();
Calendar cal = Calendar.getInstance();
cal.setTimeInMillis(0); // start at EPOCH

int days = 0
while (cal.getTimeInMillis() < now.getTimeInMillis()) {
  days += 1
  cal.add(Calendar.DAY_OF_MONTH, 1) // increment one day at a time
}
System.out.println("Days since EPOCH = " + days);

回答7:

I wouldn't expect there to be an elegant way of doing it since it is not a very common requirement. I can't help but wonder why you want to do it...

But anyway, the way I would do it is to subtract the epoch date from the Calendar and then get the fields you want:

Calendar timeSinceEpoch = Calendar.getInstance();
timeSinceEpoch.add(Calendar.YEAR, -1970);

int yearsSinceEpoch = timeSinceEpoch.get(Calendar.YEAR);
int monthsSinceEpoch = timeSinceEpoch.get(Calendar.MONTH) + 12 * yearsSinceEpoch;