Time difference in AM/PM in Java

Question

I am trying to calculate the time difference in java by input time format in 12 hours it works well when i input start time 11:58:10 pm and end time 12:02:15 am. But as i enter 12:00:00 am and 12:00:00 pm it give difference 0 minutes. don't know why. Below is my code please let me know where i am wrong.

Scanner input = new Scanner(System.in);
System.out.print("Enter start time (HH:mm:ss aa): ");
String starttime = input.nextLine();

System.out.print("Enter end time (HH:mm:ss aa): ");
String endtime = input.nextLine();

//HH converts hour in 24 hours format (0-23), day calculation
SimpleDateFormat format = new SimpleDateFormat("HH:mm:ss aa");

Date d1 = null;
Date d2 = null;

try {
    d1 = format.parse(starttime);
    d2 = format.parse(endtime);

    //in milliseconds
    long diff = d2.getTime() - d1.getTime();

    long diffSeconds = diff / 1000 % 60;
    long diffMinutes = diff / (60 * 1000) % 60;

    System.out.print(diffMinutes + " minutes and "+diffSeconds + " seconds.");
} catch (Exception e) {
    System.out.println("Invalid fromat");
}

Answer 1

The new java.time methods LocalTime, DateTimeFormatter and Duration provide much better methods for handling this. For example:

DateTimeFormatter format = DateTimeFormatter.ofPattern("h:m:s a");

LocalTime time1 = LocalTime.parse("12:00:00 am", format);
LocalTime time2 = LocalTime.parse("2:00:20 pm", format);

Duration dur = Duration.between(time1, time2);

System.out.println(dur.toMinutes() + " minutes " + dur.toSecondsPart() + " seconds");

Note: Duration.toSecondsPart requires Java 9 or later, the rest of this code requires Java 8 or later.

Answer 2

The problem is with the following line:

SimpleDateFormat format = new SimpleDateFormat("HH:mm:ss aa");

It should be:

SimpleDateFormat format = new SimpleDateFormat("hh:mm:ss aa");

The symbol, H is used for a time in 24-hour format while h is used for that in 12-hour format.

Your calculation of diffMinutes is also wrong.

Do it as follows:

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter start time (hh:mm:ss aa): ");
        String starttime = input.nextLine();

        System.out.print("Enter end time (hh:mm:ss aa): ");
        String endtime = input.nextLine();

        SimpleDateFormat format = new SimpleDateFormat("hh:mm:ss aa");

        Date d1 = null;
        Date d2 = null;

        try {
            d1 = format.parse(starttime);
            d2 = format.parse(endtime);

            // in milliseconds
            long diff = Math.abs(d2.getTime() - d1.getTime());

            long diffSeconds = (diff / 1000) % 60;
            long diffMinutes = (diff / (60 * 1000));

            System.out.print(diffMinutes + " minutes and " + diffSeconds + " seconds.");
        } catch (Exception e) {
            System.out.println("Invalid fromat");
        }
    }
}

A sample run:

Enter start time (hh:mm:ss aa): 10:20:30 am
Enter end time (hh:mm:ss aa): 10:20:13 pm
719 minutes and 43 seconds.

Notes:

1. As @greg-449 has mentioned, you should strive to use modern date-time API.
2. The difference between two quantities is an absolute value which is always positive i.e. the difference between 2 and 5 = difference between 5 and 2 = 3. Math.abs gives you the absolute value of a number which suites best for finding the difference between two quantities.
3. You need to understand that without telling about the date, the difference between two times is always considered for the same date i.e. the difference between 12:02:15 am and 11:58:10 pm = difference between 11:58:10 pm and 12:02:15 am = 1435 minutes and 55 seconds. The difference between 11:58:10 pm on one date and 12:02:15 am on the next date is 4 minutes 5 seconds. However, your input is only for times and does not have date element and therefore the difference has been considered for the same date. Given below is the program to consider a date with time.

Program considering a date with time:

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter start time (dd.MM.yyyy at hh:mm:ss aa): ");
        String starttime = input.nextLine();

        System.out.print("Enter end time (dd.MM.yyyy at hh:mm:ss aa): ");
        String endtime = input.nextLine();

        SimpleDateFormat format = new SimpleDateFormat("dd.MM.yyyy 'at' hh:mm:ss aa");

        Date d1 = null;
        Date d2 = null;

        try {
            d1 = format.parse(starttime);
            d2 = format.parse(endtime);

            // in milliseconds
            long diff = Math.abs(d2.getTime() - d1.getTime());

            long diffSeconds = (diff / 1000) % 60;
            long diffMinutes = (diff / (60 * 1000));

            System.out.print(diffMinutes + " minutes and " + diffSeconds + " seconds.");
        } catch (Exception e) {
            System.out.println("Invalid fromat");
        }
    }
}

A sample run:

Enter start time (dd.MM.yyyy at hh:mm:ss aa): 03.03.2020 at 11:58:10 pm
Enter end time (dd.MM.yyyy at hh:mm:ss aa): 04.03.2020 at 12:02:15 am
4 minutes and 5 seconds.