Date and Calendar Classes in Java

Before Java 8, dates and times were handled by the java.util.Date and java.util.Calendar classes, together with SimpleDateFormat for formatting. These are now called the legacy date classes. You will still meet them in older code and in some libraries, so it's worth knowing how they work, and especially why they caused so many bugs. Since Java 8, the java.time package replaces them. In this lesson, you will learn the legacy classes and their problems.

The Date Class

java.util.Date represents an instant in time, stored as the number of milliseconds since 1 January 1970, 00:00 UTC (called the epoch). Despite its name, it has no separate date or time zone; it only prints itself using the computer's default time zone.

The Calendar Class

java.util.Calendar was added in Java 1.1 to work with fields such as year, month and day, and to do date arithmetic. GregorianCalendar is its usual implementation.

Example: Using Date and Calendar

Let's see an example that also shows two famous traps. The default time zone is set at the start so the output is the same on every computer.

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.TimeZone;
 
public class Legacy {
    public static void main(String[] args) {
        TimeZone.setDefault(TimeZone.getTimeZone("Asia/Kolkata"));
 
        Date epoch = new Date(0);
        System.out.println("Epoch: " + epoch);
 
        Calendar cal = Calendar.getInstance();
        cal.clear();
        cal.set(2026, 11, 25);
        System.out.println("set(2026, 11, 25) gives month "
            + (cal.get(Calendar.MONTH) + 1) + ": " + cal.getTime());
 
        cal.add(Calendar.DAY_OF_MONTH, 10);
        SimpleDateFormat fmt = new SimpleDateFormat("dd-MM-yyyy");
        System.out.println("Ten days later: " + fmt.format(cal.getTime()));
 
        Date d = cal.getTime();
        d.setTime(0);
        System.out.println("Calendar after changing the Date: "
            + fmt.format(cal.getTime()));
    }
}

Epoch: Thu Jan 01 05:30:00 IST 1970
set(2026, 11, 25) gives month 12: Fri Dec 25 00:00:00 IST 2026
Ten days later: 04-01-2027
Calendar after changing the Date: 04-01-2027

Explanation:

• new Date(0) is the epoch, printed in India Standard Time, 5 hours 30 minutes ahead of UTC.
• cal.set(2026, 11, 25) means December 25, because months count from 0. Writing 12 for December is a classic bug.
• add correctly rolls over into the next year.
• getTime() returns a new Date copy, so changing it doesn't affect the calendar. But Date itself is mutable, which makes sharing dates risky.

Problems with the Legacy Classes

Common Mistakes

Passing 12 for December to Calendar.set. That's January of the next year; use Calendar.DECEMBER.

Sharing one SimpleDateFormat between threads. It corrupts results; use DateTimeFormatter, which is immutable.

Using date.getYear(). It's deprecated and returns years since 1900.

Practice

1. Easy: Use Calendar to print the day of the week of 1 January 2027.
2. Harder: Rewrite the example using LocalDate from java.time and compare how many traps disappear.

Key Takeaways