Kotlin DateTimeParseException

You do not need any formatter to parse your input string

You input string, 2006-03-25T10:30:00+12:00 is already in the default format used by OffsetDateTime#parse and therefore, you do not need to use a formatter explicitly in order to parse your input date-time string.

m specifies the minute while M specifies the month

You have wrongly used m for the month for which the symbol is M. Check the documentation page to learn more about these symbols.

Prefer u to y

y specifies the year-of-era (era is specified as AD or BC) and is always a positive number whereas u specifies the year which is a signed (+/-) number. Normally, we do not use + sign to write a positive number but we always specify a negative number with a - sign. The same rule applies for a year. As long as you are going to use a year of the era, AD (which is mostly the case), both, y and u will give you the same number. However, the difference occurs when you use a year of the era, BC e.g. the year-of-era, 1 BC is specified as year, 0; the year-of-era, 2 BC is specified as year, -1 and so on. You can understand it better with the following demo:

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

public class Testing {
    public static void main(String[] args) {
        System.out.println(LocalDate.of(-1, 1, 1).format(DateTimeFormatter.ofPattern("u M d")));
        System.out.println(LocalDate.of(-1, 1, 1).format(DateTimeFormatter.ofPattern("y M d")));
        System.out.println(LocalDate.of(-1, 1, 1).format(DateTimeFormatter.ofPattern("yG M d")));

        System.out.println();

        System.out.println(LocalDate.of(0, 1, 1).format(DateTimeFormatter.ofPattern("u M d")));
        System.out.println(LocalDate.of(0, 1, 1).format(DateTimeFormatter.ofPattern("y M d")));
        System.out.println(LocalDate.of(0, 1, 1).format(DateTimeFormatter.ofPattern("yG M d")));

        System.out.println();

        System.out.println(LocalDate.of(1, 1, 1).format(DateTimeFormatter.ofPattern("u M d")));
        System.out.println(LocalDate.of(1, 1, 1).format(DateTimeFormatter.ofPattern("y M d")));
        System.out.println(LocalDate.of(1, 1, 1).format(DateTimeFormatter.ofPattern("yG M d")));
    }
}

Output:

-1 1 1
2 1 1
2BC 1 1

0 1 1
1 1 1
1BC 1 1

1 1 1
1 1 1
1AD 1 1

Note: I’ve used Java to demonstrate the solution but it will also work in Kotlin.

The final solution:

import java.time.OffsetDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        OffsetDateTime odt = OffsetDateTime.parse("2006-03-25T10:30:00+12:00");
        System.out.println(odt);

        // Format it into the desired pattern
        DateTimeFormatter dtf = DateTimeFormatter.ofPattern("dd, MM, uuuu", Locale.US);
        String formatted = dtf.format(odt);
        System.out.println(formatted);
    }
}

Output:

2006-03-25T10:30+12:00
25, 03, 2006

Learn more about the modern date-time API from Trail: Date Time.