[КЛ@ВИШ_nick]

<div id = "time">40 мин</div><button id = "plus">+</button>
<script>
var divTime = document.getElementById("time"),
	minutes = parseInt(divTime.innerHTML) ;
document.getElementById("plus").addEventListener("click", function() {
	minutes += 5 ;
	var text = minutes + " мин" ;
	if(minutes >= 60) {
		text = (minutes / 60 | 0) + " ч " + minutes % 60 + " мин" ;
	} ;
	divTime.innerHTML = text ;
}) ;
</script>