Use js to implement button control text box plus 1 minus 1 applied to hours + minutes

function add_num(a,b,c){

if(c<a){
c++;
}
else{
c=b;
}
if(c<=9&&c>=0){
c="0"+c;
}
return c;
}
function sub_num(a,b,c){
if(c>a){
c--;
}
else{
c=b;
}
if(c<=9&&c>=0){
c="0"+c;
}
return c;
}
function add_hour(){
var current_num=$("#hour").attr("value");
current_num=add_num(23,0,current_num);
$("#hour").attr("value",current_num);
}
function sub_hour(){
var current_num=$("#hour").attr("value");
current_num=sub_num(0,23,current_num);
$("#hour").attr("value",current_num);
}
function add_minute(){
var current_num=$("#minute").attr("value");
current_num=add_num(59,0,current_num);
$("#minute").attr("value",current_num);
}
function sub_minute(){
var current_num=$("#minute").attr("value");
current_num=sub_num(0,59,current_num);
$("#minute").attr("value",current_num);
}