A Beginner's Guide to ISO 8601–1: Understanding Date and Time Formatting

In the world of software development, handling date and time can be tricky. Different regions use different formats, leading to confusion and errors. That’s where ISO 8601–1 comes in — a universal standard for representing date and time.

In this guide, we’ll break down ISO 8601–1, explore its format, and provide Kotlin and Java examples to help you implement it in your applications.

What is ISO 8601–1?

ISO 8601–1 provides a structured format for date-time representation to ensure consistency across systems.

Standard Format:

YYYY-MM-DDTHH:mm:ss±hh:mm

Example:

2024-02-26T14:30:00+08:00

Breaking Down the Format:

• YYYY-MM-DD → Date (2024-02-26)
• T → Separator between date and time
• HH:mm:ss → Time in 24-hour format (14:30:00)
• ±hh:mm → Time zone offset (+08:00)

Comparing Java/Kotlin Date-Time Classes

Before we dive into formatting and parsing, let’s compare the different date-time classes available in java.time:

Date-Time Classes Comparison

General Rule:

• Use ZonedDateTime for scheduling events where the time zone matters.
• Use OffsetDateTime for API responses where only offset is needed.
• Use LocalDateTime for database storage when the time zone is handled separately.
• Use Instant for UTC timestamps that need to be time zone agnostic.

Formatting & Parsing ISO 8601–1 Date-Time

Each is explained with a Kotlin and Java example that can be tested using the following:

• Kotlin Playground
• Online Java Compiler

1. Formatting Current Date-Time in ISO 8601–1

Kotlin Example:

import java.time.ZonedDateTime
import java.time.format.DateTimeFormatter

fun main() {
    val now = ZonedDateTime.now()
    val formattedDate = now.format(DateTimeFormatter.ISO_OFFSET_DATE_TIME)
    println("Formatted Date-Time: $formattedDate")
}

// Sample Output
// Formatted Date-Time: 2025-03-04T09:59:56.832948787Z

Java Example:

import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;

public class ISO8601Formatter {
    public static void main(String[] args) {
        ZonedDateTime now = ZonedDateTime.now();
        String formattedDate = now.format(DateTimeFormatter.ISO_OFFSET_DATE_TIME);
        System.out.println("Formatted Date-Time: " + formattedDate);
    }
}

// Sample Output
// Formatted Date-Time: 2025-03-04T10:00:49.861977921Z

2. Parsing an ISO 8601–1 Date-Time String

Kotlin Example:

import java.time.ZonedDateTime
import java.time.format.DateTimeFormatter

fun main() {
    val dateString = "2024-02-26T14:30:00+08:00"
    val parsedDate = ZonedDateTime.parse(dateString, DateTimeFormatter.ISO_OFFSET_DATE_TIME)
    println("Parsed Date: $parsedDate")
}

// Sample Output
// Parsed Date: 2024-02-26T14:30+08:00

Java Example:

import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;

public class ParseISO8601 {
    public static void main(String[] args) {
        String dateString = "2024-02-26T14:30:00+08:00";
        ZonedDateTime parsedDate = ZonedDateTime.parse(dateString, DateTimeFormatter.ISO_OFFSET_DATE_TIME);
        System.out.println("Parsed Date: " + parsedDate);
    }
}

// Sample Output
// Parsed Date: 2024-02-26T14:30+08:00

Common Pitfalls & Mistakes to Avoid

1. Ignoring Time Zone Information

Kotlin Wrong Approach:

import java.time.ZonedDateTime
import java.time.format.DateTimeFormatter

fun main() {
    val now = ZonedDateTime.now()
    val formattedDate = now.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME)
    println("Formatted Date-Time: $formattedDate")
}

// Sample Output
// Formatted Date-Time: 2025-03-04T10:23:00.24538642

Java Wrong Approach:

import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;

public class ISO8601Formatter {
    public static void main(String[] args) {
        ZonedDateTime now = ZonedDateTime.now();
        String formattedDate = now.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME);
        System.out.println("Formatted Date-Time: " + formattedDate);
    }
}

// Sample Output
// Formatted Date-Time: 2025-03-04T10:24:04.454313402

Fix: Always use ISO_OFFSET_DATE_TIME to include the time zone.

2. Using SimpleDateFormat Instead of java.time API

Wrong Approach:

SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssXXX")

Fix: Use DateTimeFormatter from java.time.

3. Not Including ‘T’ Separator in API Responses

Incorrect API Response:

"timestamp": "2024-02-26 14:30:00+08:00"

Correct API Response:

"timestamp": "2024-02-26T14:30:00+08:00"

Conclusion

ISO 8601–1 provides a standardized and reliable way to represent date-time values, eliminating ambiguity and ensuring seamless integration across systems. Selecting the right Java/Kotlin date-time class isn’t just a technical choice — it’s a crucial step in preventing errors and improving the robustness of your applications. By making informed decisions about date-time handling, you can build software that is both accurate and future-proof.