Module 9: Date & Time API (java.time)

Phase

Phase 3 – Advanced Concepts

Target Audience

• Java developers tired of Date and Calendar
• Backend developers handling time zones and formatting
• Anyone writing production-grade Java applications

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1. Objective of This Module

By the end of this module, you will:

• Understand problems with old Date/Calendar APIs
• Use the modern java.time API confidently
• Handle dates, times, and time zones correctly
• Format and parse dates safely

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2. Problems with Old Date & Calendar API

Before Java 8:

• java.util.Date was mutable
• Confusing month indexing (0-based)
• Poor timezone handling
• Not thread-safe
• Hard to read and error-prone

Date date = new Date();

This gives date + time + timezone all mixed together.

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3. Introduction to java.time Package

Java 8 introduced java.time, inspired by Joda-Time.

Key principles:

• Immutable classes
• Clear separation of date, time, and timezone
• Thread-safe by design

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4. Core Date & Time Classes

LocalDate

Represents date only (no time, no timezone).

LocalDate today = LocalDate.now();
LocalDate dob = LocalDate.of(1995, 5, 20);

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LocalTime

Represents time only.

LocalTime now = LocalTime.now();
LocalTime meeting = LocalTime.of(10, 30);

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LocalDateTime

Represents date + time (no timezone).

LocalDateTime dateTime = LocalDateTime.now();

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5. Working with Time Zones

ZonedDateTime

Represents date + time + timezone.

ZonedDateTime zoned =
    ZonedDateTime.now(ZoneId.of("Asia/Kolkata"));

Convert between zones:

ZonedDateTime usTime =
    zoned.withZoneSameInstant(ZoneId.of("America/New_York"));

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6. Period & Duration

Period

Used for date-based differences.

Period age =
    Period.between(dob, LocalDate.now());

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Duration

Used for time-based differences.

Duration duration =
    Duration.between(startTime, endTime);

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7. Formatting & Parsing Dates

DateTimeFormatter

DateTimeFormatter formatter =
    DateTimeFormatter.ofPattern("dd-MM-yyyy");

String formatted =
    LocalDate.now().format(formatter);

LocalDate parsed =
    LocalDate.parse("21-01-2026", formatter);

Thread-safe and reusable.

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8. Real-World Use Cases

• Age calculation
• Booking systems
• Logging timestamps
• Timezone conversion for global apps

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9. Common Mistakes

• Mixing old Date with java.time
• Ignoring time zones
• Using LocalDateTime when timezone is required
• Hardcoding date formats

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10. Hands-On Exercises

Exercise 1

Calculate age using LocalDate and Period.

Exercise 2

Convert IST time to US time.

Exercise 3

Parse and format dates using custom patterns.

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11. Summary

• java.time is immutable and thread-safe
• Choose the right class for the problem
• Always be explicit about time zones

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12. What’s Next?

➡️ Module 10: Parallel Streams & Performance