var Min = 660;
var Step = 30

function setTime($input) {
	var hour = Math.floor(Min/60);
	var min = Min % 60;
	
	if (min < 10) {
		min = '0' + min;
	}
	
	if (hour < 10) {
		hour = '0' + hour;
	}
	
	$input.val(hour + ':' + min);
}

$('.decrease').click(function () {
	Min -= Step;
	if (Min <= 0) Min += 1440;
	setTime($(this).parent().find('input'));
});

$('.increase').click(function () {
	Min += Step;
	if (Min >= 1440) Min -= 1440;
	setTime($(this).parent().find('input'));
});