Date Class in Java – java.util.Date and the Modern java.time API

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;
import java.util.TimeZone;

public class DateBasics {
    public static void main(String[] args) throws Exception {
        Date now = new Date();                                   // current instant
        System.out.println("new Date() is close to currentTimeMillis: "
                + (Math.abs(now.getTime() - System.currentTimeMillis()) < 1000));

        Date launch = new Date(1_791_000_000_000L);              // milliseconds since 1970-01-01T00:00Z
        System.out.println("getTime()      : " + launch.getTime());

        SimpleDateFormat fmt = new SimpleDateFormat("dd-MM-yyyy HH:mm:ss zzz", Locale.ENGLISH);
        fmt.setTimeZone(TimeZone.getTimeZone("Asia/Kolkata"));   // Date has no time zone itself
        System.out.println("formatted (IST): " + fmt.format(launch));
        fmt.setTimeZone(TimeZone.getTimeZone("UTC"));
        System.out.println("formatted (UTC): " + fmt.format(launch));

        SimpleDateFormat parser = new SimpleDateFormat("yyyy-MM-dd");
        parser.setTimeZone(TimeZone.getTimeZone("UTC"));
        Date d = parser.parse("2026-10-11");
        System.out.println("parsed millis  : " + d.getTime());

        System.out.println("before/after   : " + d.before(launch) + " / " + d.after(launch));
        System.out.println("compareTo      : " + d.compareTo(launch));
    }
}

import java.util.Date;

public class DateProblems {
    @SuppressWarnings("deprecation")
    public static void main(String[] args) {
        // 1. Confusing, deprecated constructor: year is offset from 1900, month is 0-based
        Date d = new Date(126, 9, 11);          // means 2026-10-11!
        System.out.println("getYear() = " + d.getYear() + ", getMonth() = " + d.getMonth());

        // 2. Mutable: anyone holding the reference can change it
        Date deadline = new Date(0);
        Date alias = deadline;
        alias.setTime(86_400_000L);             // one day later
        System.out.println("deadline changed through alias: " + (deadline.getTime() == 86_400_000L));
    }
}

import java.time.*;
import java.time.format.DateTimeFormatter;
import java.time.temporal.ChronoUnit;
import java.util.Date;
import java.util.Locale;

public class ModernDateTime {
    public static void main(String[] args) {
        LocalDate date = LocalDate.of(2026, 10, 11);                 // date only, immutable
        LocalDate due = date.plusDays(30);
        System.out.println("date + 30 days : " + due + " (" + due.getDayOfWeek() + ")");
        System.out.println("days between   : " + ChronoUnit.DAYS.between(date, due));

        LocalDateTime meeting = LocalDateTime.of(2026, 10, 11, 15, 30);
        ZonedDateTime ist = meeting.atZone(ZoneId.of("Asia/Kolkata"));
        System.out.println("IST            : " + ist);
        System.out.println("New York       : " + ist.withZoneSameInstant(ZoneId.of("America/New_York")));

        DateTimeFormatter f = DateTimeFormatter.ofPattern("dd MMM yyyy, hh:mm a", Locale.ENGLISH);
        System.out.println("formatted      : " + meeting.format(f));
        System.out.println("parsed         : " + LocalDate.parse("2026-12-25").getDayOfWeek());

        // Converting between legacy Date and java.time
        Instant instant = ist.toInstant();
        Date legacy = Date.from(instant);
        System.out.println("round trip     : " + legacy.toInstant().equals(instant));
    }
}