Date & Time

Java has two APIs for dates and times: java.util.Date/Calendar (legacy, avoid) and java.time (modern, recommended since Java 8). The java.time API was designed based on Joda-Time and fixes all the shortcomings of the old API — it is immutable, thread-safe, and has intuitive methods.

Key classes in java.time: LocalDate (date only), LocalTime (time only), LocalDateTime (date + time), ZonedDateTime (date + time + time zone), Instant (timestamp), and Duration/Period (time intervals). All of these classes are immutable — methods such as plusDays() return a new object.

DateTimeFormatter is used to format a date into a string, and LocalDate.parse() parses a string into a date. Format patterns use specific letters: yyyy (year), MM (month), dd (day), HH (hour), mm (minute), ss (second).

Date-Time Classes:

Code Example

import java.time.*;
import java.time.format.DateTimeFormatter;
import java.time.temporal.ChronoUnit;

public class DateTime {
    public static void main(String[] args) {
        // Current date and time
        LocalDate today = LocalDate.now();
        LocalTime now = LocalTime.now();
        LocalDateTime datetime = LocalDateTime.now();
        System.out.println("Date: " + today);
        System.out.println("Time: " + now);
        System.out.println("DateTime: " + datetime);

        // Create a specific date
        LocalDate birthday = LocalDate.of(2000, 6, 15);
        System.out.println("Birthday: " + birthday);

        // Date operations
        LocalDate tomorrow = today.plusDays(1);
        LocalDate mingguDepan = today.plusWeeks(1);
        long daysUntilBirthday = today.until(birthday, ChronoUnit.DAYS);
        System.out.println("Tomorrow: " + tomorrow);
        System.out.println("Days until birthday: " + Math.abs(daysUntilBirthday));

        // Format
        DateTimeFormatter fmt = DateTimeFormatter.ofPattern("dd MMMM yyyy");
        System.out.println("Formatted: " + today.format(fmt));

        // Parsing
        LocalDate parsed = LocalDate.parse("2026-12-25");
        System.out.println("Parsed: " + parsed);

        // Period
        Period age = Period.between(birthday, today);
        System.out.println("Age: " + age.getYears() + " years, "
            + age.getMonths() + " months");
    }
}

Code Explanation

1. LocalDate.now() gets the current system date. LocalTime.now() and LocalDateTime.now() get the time and the combination of both.
2. LocalDate.of(year, month, day) creates a specific date. Months use numbers (1-12), not names.
3. plusDays(), plusWeeks(), plusMonths() return a new LocalDate (immutable).
4. until() computes the interval between two dates in a given unit.
5. DateTimeFormatter.ofPattern() defines the output format. dd for the day, MMMM for the full month name, yyyy for the year.
6. Period.between() computes an interval in units of years, months, and days.

Everyday Example

A bill due date reminder program:

import java.time.LocalDate;
import java.time.temporal.ChronoUnit;

public class PengingatTagihan {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();
        LocalDate dueDate = LocalDate.of(2026, 7, 20);

        long daysRemaining = ChronoUnit.DAYS.between(today, dueDate);

        System.out.println("Today: " + today);
        System.out.println("Due date: " + dueDate);

        if (daysRemaining < 0) {
            System.out.println("BILL IS OVERDUE!");
        } else if (daysRemaining <= 3) {
            System.out.println("PAY NOW! Only " + daysRemaining + " days left");
        } else {
            System.out.println("You still have " + daysRemaining + " days");
        }
    }
}

Summary

The java.time API is the modern standard for dates and times in Java. Always use these classes rather than the outdated Date/Calendar. All java.time classes are immutable and thread-safe, so they are safe to use anywhere without worrying about concurrent modification.