10.3 Date & Time Formatting (DateTimeFormatter)

We have finished the internal date and time computations. Now comes the final gateway: "Creating human-readable date strings, or conversely, converting strings back into date objects."

For this, we use the DateTimeFormatter class.

1. Date Object → String Conversion: format()

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

LocalDateTime now = LocalDateTime.now();

// Define the desired date format pattern
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");

// Convert the LocalDateTime object into a formatted string
String formatted = now.format(formatter);
System.out.println(formatted); // 2026-03-13 17:30:08

Key Pattern Characters

Pattern	Meaning	Example
yyyy	Year (4 digits)	2026
yy	Year (2 digits)	26
MM	Month (2 digits, 01~12)	03
M	Month (1-2 digits, 1~12)	3
dd	Day (2 digits, 01~31)	07
d	Day (1-2 digits, 1~31)	7
HH	Hour (24-hour clock, 00~23)	17
hh	Hour (12-hour clock, 01~12)	05
mm	Minutes (00~59)	30
ss	Seconds (00~59)	08
E	Day of week (abbreviated)	Thu
EEEE	Day of week (full name)	Thursday

Let's print dates in a variety of formats:

LocalDateTime now = LocalDateTime.now();

// US-style date notation
DateTimeFormatter us = DateTimeFormatter.ofPattern("MM/dd/yyyy");
System.out.println(now.format(us)); // 03/13/2026

// Standard format with day of week
DateTimeFormatter withDay = DateTimeFormatter.ofPattern("yyyy-MM-dd (EEEE)");
System.out.println(now.format(withDay)); // 2026-03-13 (Thursday)

// ISO 8601 standard format (frequently used in API communication)
DateTimeFormatter iso = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss");
System.out.println(now.format(iso)); // 2026-03-13T17:30:08

2. String → Date Object Conversion: parse()

Conversely, to convert a textual date string like "2026-03-13" back into a LocalDate object, use the parse() method. You will use this essentially whenever you receive dates as raw text from web forms or external APIs.

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;

String dateString = "03/13/2026";

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

// Parse the string into a LocalDate object
LocalDate parsedDate = LocalDate.parse(dateString, formatter);

System.out.println("Parsed Date: " + parsedDate); // 2026-03-13
System.out.println("Year: " + parsedDate.getYear());         // 2026
System.out.println("Month: " + parsedDate.getMonthValue());  // 3
System.out.println("Day: " + parsedDate.getDayOfMonth());    // 13

With this, you have now mastered all the core skills for handling dates and times in Java!

• LocalDate / LocalTime / LocalDateTime: Creating date/time objects
• Period / Duration: Calculating intervals between dates/times
• DateTimeFormatter: Bidirectional conversion between date/time objects and strings

In the next Phase, we will dive into Java's Collection Framework (List, Set, Map)!