Module 4: LocalDateTime

Learning Objectives

• Understand what LocalDateTime objects are
• Know how to use LocalDateTime to:
  • Get the current date and time
  • Create an object representative of a specific date
• Know how to compare between other times and dates
• Know how to add and remove amounts of time from a LocalDateTime object
• Understand ZonedDateTime and how to covert between LocalDateTime to compensate for timezones

Working with LocalDateTime

In Java, the LocalDateTime class represents a date-time without a timezone, like 2022-04-25T10:15:30. It's part of the java.time package introduced in Java 8, which provides a comprehensive date and time API that is both simple to use and powerful.

Creating LocalDateTime Objects

There are several ways to create LocalDateTime objects:

// Get the current date and time
LocalDateTime now = LocalDateTime.now();

// Create a specific date and time
LocalDateTime dateTime = LocalDateTime.of(2022, 4, 25, 10, 15, 30);

// Parse a string to LocalDateTime
LocalDateTime parsed = LocalDateTime.parse("2022-04-25T10:15:30");

Comparing LocalDateTime Objects

LocalDateTime provides methods for comparing date-time objects:

LocalDateTime now = LocalDateTime.now();
LocalDateTime pastDate = now.minusDays(5);
LocalDateTime futureDate = now.plusDays(5);

// Check if now is after pastDate
boolean isAfter = now.isAfter(pastDate);  // true

// Check if now is before futureDate
boolean isBefore = now.isBefore(futureDate);  // true

// Check if two dates are equal
boolean isEqual = now.isEqual(now);  // true

Manipulating LocalDateTime Objects

LocalDateTime objects are immutable, but you can create new objects with modifications:

LocalDateTime now = LocalDateTime.now();

// Adding time
LocalDateTime future = now.plusYears(1)
                          .plusMonths(2)
                          .plusDays(3)
                          .plusHours(4)
                          .plusMinutes(5)
                          .plusSeconds(6);

// Subtracting time
LocalDateTime past = now.minusYears(1)
                        .minusMonths(2)
                        .minusDays(3)
                        .minusHours(4)
                        .minusMinutes(5)
                        .minusSeconds(6);

// With specific field values
LocalDateTime withFields = now.withYear(2025)
                             .withMonth(Month.JULY.getValue())
                             .withDayOfMonth(15)
                             .withHour(10)
                             .withMinute(30)
                             .withSecond(0);

Formatting LocalDateTime

You can format LocalDateTime objects in various ways:

LocalDateTime dateTime = LocalDateTime.of(2022, 4, 25, 10, 15, 30);

// Using predefined formats
String iso = dateTime.toString();  // 2022-04-25T10:15:30

// Using DateTimeFormatter
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd-MM-yyyy HH:mm:ss");
String formatted = dateTime.format(formatter);  // 25-04-2022 10:15:30

// Other formatting examples
String formatDay = dateTime.format(DateTimeFormatter.ofPattern("EEEE, MMMM dd, yyyy"));  // Monday, April 25, 2022
String formatTime = dateTime.format(DateTimeFormatter.ofPattern("hh:mm a"));  // 10:15 AM

Working with Time Zones

LocalDateTime doesn't have timezone information. For timezone-aware date-time operations, you can convert to ZonedDateTime:

LocalDateTime localDateTime = LocalDateTime.now();

// Convert to a specific zone
ZoneId newYorkZone = ZoneId.of("America/New_York");
ZonedDateTime newYorkTime = localDateTime.atZone(newYorkZone);

// Convert to a different zone
ZoneId tokyoZone = ZoneId.of("Asia/Tokyo");
ZonedDateTime tokyoTime = newYorkTime.withZoneSameInstant(tokyoZone);

// Get the offset
ZoneOffset offset = tokyoTime.getOffset();

// Display with timezone
String zonedFormatted = tokyoTime.format(
    DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss z")
);  // 2022-04-26 00:15:30 JST