Use classes in the java.time package to replace the old Date and Calendar classes; 2. Get the current date and time through LocalDate, LocalDateTime, and LocalTime; 3. Create a specific date and time using the of() method; 4. Use the plus/minus method to immutably increase and decrease the time; 5. Use ZonedDateTime and ZoneId to process the time zone; 6. Format and parse date strings through DateTimeFormatter; 7. Use Instant to be compatible with the old date types when necessary; date processing in modern Java should give priority to the use of the java.time API, which provides a clear, immutable and thread-safe interface, suitable for most practical scenarios, and can effectively avoid the defects of traditional date classes.

Working with calendars in Java can be confusing if you're using outdated methods, but modern Java (especially from Java 8 onward) provides a much cleaner and independent way to handle dates and times via the java.time package. Here's a practical guide on how to work with calendars effectively.

? Use java.time instead of Old Date/Calendar Classes

Avoid the old java.util.Date and java.util.Calendar classes — they're mutable, not thread-safe, and hard to work with. Instead, use the java.time API introduced in Java 8 .

Key classes:

• LocalDateTime – Date and time without timezone
• ZonedDateTime – Date and time with timezone
• LocalDate – Just a date (eg, 2025-04-05)
• LocalTime – Just a time (eg, 14:30)
• YearMonth , MonthDay , etc. – Specialized date types

? 1. Get Current Date and Time

 import java.time.LocalDateTime;
import java.time.LocalDate;
import java.time.LocalTime;

LocalDateTime now = LocalDateTime.now();
System.out.println("Now: " now);

LocalDate today = LocalDate.now();
System.out.println("Today: " today);

LocalTime time = LocalTime.now();
System.out.println("Current time: " time);

This gives you clean, immutable objects representing the current moment.

?? 2. Create Specific Dates

You can create fixed dates easily:

 LocalDate birthday = LocalDate.of(1990, 5, 15); // Year, Month, Day
// Or use Month enum
LocalDate xmas = LocalDate.of(2025, java.time.Month.DECEMBER, 25);

Similarly for time:

 LocalTime meetingTime = LocalTime.of(14, 30); // 2:30 PM

Combine date and time:

 LocalDateTime event = LocalDateTime.of(2025, 3, 20, 9, 0); // Mar 20, 9:00 AM

? 3. Add or Subtract Time (Immutably)

All java.time objects are immutable , so you get a new instance when modifying.

 LocalDate today = LocalDate.now();
LocalDate nextWeek = today.plusWeeks(1);
LocalDate nextMonth = today.plusMonths(1);
LocalDate tenDaysAgo = today.minusDays(10);

System.out.println("Next week: " nextWeek);

Works similarly for LocalDateTime , LocalTime , etc.

? 4. Work With Time Zones (ZonedDateTime)

If you need time zones (eg, scheduled across regions):

 import java.time.ZoneId;
import java.time.ZonedDateTime;

ZoneId nyZone = ZoneId.of("America/New_York");
ZonedDateTime nyTime = ZonedDateTime.now(nyZone);

ZoneId londonZone = ZoneId.of("Europe/London");
ZonedDateTime londonTime = ZonedDateTime.now(londonZone);

System.out.println("NY: " nyTime);
System.out.println("London: " londonTime);

You can also convert between zones:

 ZonedDateTime nyInLondonTime = nyTime.withZoneSameInstant(londonZone);

? 5. Format and Parse Dates

Convert to/from strings using DateTimeFormatter .

 import java.time.format.DateTimeFormatter;

LocalDate date = LocalDate.of(2025, 4, 5);
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd-MM-yyyy");
String formatted = date.format(formatter); // "05-04-2025"
System.out.println(formatted);

// Parse string back to date
String input = "15-08-2023";
LocalDate parsed = LocalDate.parse(input, formatter);

Common patterns:

• "yyyy-MM-dd" → 2025-04-05
• "dd/MM/yyyy HH:mm" → 05/04/2025 14:30
• "MMM dd, yyyy" → Apr 05, 2025

? 6. Legacy Compatibility (if needed)

If you must interact with old APIs that use java.util.Date :

 import java.util.Date;
import java.time.Instant;

// Convert LocalDateTime to Date
LocalDateTime ldt = LocalDateTime.now();
Date oldDate = Date.from(ldt.atZone(ZoneId.systemDefault()).toInstant());

// Convert Date to LocalDateTime
Date legacyDate = new Date();
LocalDateTime ldt2 = legacyDate.toInstant()
                               .atZone(ZoneId.systemDefault())
                               .toLocalDateTime();

But try to stick with java.time throughout your app.

? Pro Tips

• Always specify time zones explicitly when dealing with real-world events.
• Use Instant for timestamps (eg, logging, database storage).
• For recurring events or calendar logic (eg, "first Monday of the month"), consider libraries like ThreeTen-Extra or Joda-Time (though Joda is now in maintenance mode).

Basically, working with calendars in Java is clean and safe if you use java.time . Avoid the old Calendar class unless you're maintaining legacy code. Focus on LocalDate , LocalDateTime , ZonedDateTime , and formatting tools — they cover most real-world use cases.

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