Get the First day and the last day of month from an Instant

Question

I have a method using 2 Instants in parameters getIssuesBillable(Instant start, Instant end, ....), my question is how I get the first day of a month and the last day of the month using a Java 8 Instant? I already tried use withDayOfMonth(), and lengthOfMonth():

LocalDate initial = LocalDate.now();
LocalDate start = initial.withDayOfMonth(firstDayOfMonth());
LocalDate end = initial.withDayOfMonth(lastDayOfMonth());

But in this case, I need to convert and make some workarounds in this case, if someone knows a better way to do it I really appreciate any response.

Answer 1

tl;dr

Use modern java.time classes.

Here is a brief nonsensical example of starting with an Instant (a moment in UTC), assigning a time zone to view that moment through the wall-clock time used by the people of a particular region (a time zone), extracting the year-and-month as perceived in that time zone, and determining the first and last day of that month, rendering LocalDate objects.

YearMonth                                 // Represent a year-and-month, the entire month as a whole.
.from(                                    // Determine the year-and-month of some other date-time object.
    Instant                               // Represent a moment in UTC.
    .now()                                // Capture the current moment in UTC.
    .atZone(                              // Adjust from UTC to a particular time zone.
        ZoneId.of( "Pacific/Auckland" ) 
    )                                     // Returns a `ZonedDateTime` object.
)                                         // Returns a `YearMonth` object.
.atDay( 1 )                               // Returns a `LocalDate` object.

…or

…
.atEndOfMonth()                           // Returns a `LocalDate` object.

By the way, a realistic version of that particular code would be: YearMonth.now( ZoneId.of( "Pacific/Auckland" ) ).atDay( 1 )

Moment versus date-only

Instant is a moment in UTC, a date with time-of-day and an offset-from-UTC of zero.

A LocalDate is a date-only value, without time-of-day and without time zone.

You need to specify the time zone by which you want to perceive the date, the wall-clock time used by the people of a particular region.

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.

If no time zone is specified, the JVM implicitly applies its current default time zone. That default may change at any moment during runtime(!), so your results may vary. Better to specify your desired/expected time zone explicitly as an argument.

Specify a proper time zone name in the format of continent/region, such as America/Montreal, Africa/Casablanca, or Pacific/Auckland. Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!).

ZoneId z = ZoneId.of( "Africa/Casablanca" ) ;
ZonedDateTime zdt = instant.atZone( z ) ;

Extract the date-only.

LocalDate ld = zdt.toLocalDate() ;

From there, proceed with your other code.

Or work with the month as a whole, using YearMonth class.

YearMonth ym = YearMonth.from( zdt ) ;
LocalDate first = ym.atDay( 1 ) ;
LocalDate last = ym.atEndOfMonth() ;

Tip: You might find helpful the LocalDateRange and Interval classes in the ThreeTen-Extra library.

Tip: Learn about the Half-Open approach to define a span of time, where the beginning is inclusive while the ending is exclusive. So a month starts on the first and runs up to, but does not include, the first day of the following month.

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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, Java SE 10, Java SE 11, and later - 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
  • Most 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.