Java 8 : Get week number since epoch time

Question

Given LocalDate I want to convert to week number since Epoch

One way to do that is:

LocalDate date = datePicker.getValue(); // input from your date picker
Locale locale = Locale.US;
int weekOfYear = date.get(WeekFields.of(locale).weekOfWeekBasedYear());

And X = find weeks since Epoch to prevYear

And then result = X + weekOfYear

Although someway we can find out "week number since epoch", is there clean solution to find it using Java 8 ?

UPDATE: Even above solution wont work as one week(always starting from Sunday) can span across two years

Answer 1

There's a method out there for milliseconds since epoch for a Date:

long timeInMillis = date.getTime();

and apparently there are 604800000 milliseconds to a week. So I guess you could go

long weeksSinceEpoch = date.getTime() / 604800000;

I think you'd probably actually need to add 1 to your final value though, since Java truncates longs.

Answer 2

I am turning the comment by Tunaki into an Answer.

tl;dr

ChronoUnit.WEEKS.between ( 
    LocalDate.ofEpochDay ( 0 ) , 
    LocalDate.now( ZoneOffset.UTC ) 
)

LocalDate

The LocalDate class represents a date-only value without time-of-day and without time zone.

A time zone is crucial in determining a date. For any given moment, the date varies around the globe by zone. For example, a few minutes after midnight in Paris France is a new day while still “yesterday” in Montréal Québec.

Here we use UTC as the time zone for today’s date, in accordance with the epoch being defined in UTC.

LocalDate today = LocalDate.now( ZoneOffset.UTC );

ChronoUnit

The ChronoUnit class has methods for calculating an elapsed number of years or months or weeks or such. For weeks it simply takes the number of days and divides by 7. So the first day-of-week is irrelevant. Not sure if that meets your needs or not as the Question is vague.

LocalDate today = LocalDate.now ( ZoneOffset.UTC );  // Using UTC to match the definition of the Java epoch.
LocalDate epoch = LocalDate.ofEpochDay ( 0 );

long weeks = ChronoUnit.WEEKS.between ( epoch , today );

Dump to console.

System.out.println ( "epoch: " + epoch + " | today in UTC: " + today + " | weeks: " + weeks );

epoch: 1970-01-01 | today in UTC: 2016-08-29 | weeks: 2434

You could adjust your starting date to a specific day-of-week if that makes sense for your needs. I'm not sure if that fits because the Question is not clear.

LocalDate epoch = LocalDate.ofEpochDay ( 0 );
LocalDate start = epoch.with ( TemporalAdjusters.nextOrSame ( DayOfWeek.SUNDAY ) ); // Get the first Sunday that *is* the epoch or *follows* the epoch.
long weeks = ChronoUnit.WEEKS.between ( start , today );

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About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.