//package classes;

public class TimeSpan {

	private int hours;
	private int minutes;

	// Construtor to pass hours and minutes
	public TimeSpan(int hours, int minutes) {
		this.hours = 0;
		this.minutes = 0;
		add(hours, minutes);
	}

	// this methods thorws an error if invalid hours and miutes passes into the
	// argument
	public void add(int hours, int minutes) {
		if (hours < 0 || minutes < 0) {
			throw new IllegalArgumentException();
		}

		this.hours += hours;
		this.minutes += minutes;

		// converts each 60 minutes into one hour
		this.hours += this.minutes / 60;
		this.minutes = this.minutes % 60;
	}
	
	public void substract(TimeSpan span) {
	   
		if (span.hours < 0 || span.minutes < 0) {
			throw new IllegalArgumentException();
		}

		
		this.minutes = ((this.hours*60+this.minutes)-((span.hours*60)+span.minutes))%60;
		
		this.hours -= span.hours;
		
		if (this.hours < 0 || this.minutes < 0) {
			throw new IllegalArgumentException();
		}
		
		
	}

	// returns a String for this time span, such as "6h 15m"
	public String toString() {
		return hours + "h " + minutes + "m";
	}

public static void main(String[] args) {

		TimeSpan elapsed = new TimeSpan(5, 56);
		
		TimeSpan elapsed2 = new TimeSpan(2, 56);

		System.out.println("elapsed time before subtracting: " + elapsed);
		System.out.println("subtract: " + elapsed2);
		
		elapsed.substract(elapsed2);

		System.out.println("elapsed time after subtracting: " + elapsed);

	}

}

Output:

elapsed time before subtracting: 5h 56m
subtract: 2h 56m
elapsed time after subtracting: 3h 0m