Simple Timer with Intervals

Hey guys I am a student in college currently taking an intro to computer science class. I have taken a liking to it and have decided to try and learn some coding. Currently I am working on a timer basically one from online, that I want to fix up. I'll post script below for advice, anyways my goal is to create intervals on the timer so that it will have two screens, overall time and rounds time. Therefore I want to implement intervals into it so that its on a loop pattern going 30seconds, 10seconds, 30seconds until it hits 10minutes or whatever. Any advice tips or help would be much appreciated. In my class currently we are just doing html so this is all new to me.

<script language="javascript">
var base = 60;

var clocktimer,dateObj,dh,dm,ds,ms;
var readout='';
var h=1;
var m=1;
var tm=1;
var s=0;
var ts=0;
var ms=0;
var show=true;
var init=0;
var mPLUS=new Array(
'm0',
'm1',
'm2',
'm3',
'm4',
'm5',
'm6',
'm7',
'm8',
'm9'
);
var ii=0;

function clearALL() {
clearTimeout(clocktimer);
h=1;m=1;tm=1;s=0;ts=0;ms=0;
init=0;show=true;
readout='00:00:00.00';
document.clockform.clock.value=readout;
var CF = document.clockform;
for (ij=0;ij<=9;ij++) { CF[mPLUS[ij]].value = ''; }
ii = 0;
}

function addMEM() {
if (init>0) {
var CF = document.clockform;
CF[mPLUS[ii]].value = readout;
if (ii==9) { ii = 0; } else { ii++; }
}
}

function startTIME() {

var cdateObj = new Date();
var t = (cdateObj.getTime() - dateObj.getTime())-(s*1000);

if (t>999) { s++; }

if (s>=(m*base)) {
ts=0;
m++;
} else {
ts=parseInt((ms/100)+s);
if(ts>=base) { ts=ts-((m-1)*base); }
}

if (m>(h*base)) {
tm=1;
h++;
} else {
tm=parseInt((ms/100)+m);
if(tm>=base) { tm=tm-((h-1)*base); }
}

ms = Math.round(t/10);
if (ms>99) {ms=0;}
if (ms==0) {ms='00';}
if (ms>0&&ms<=9) { ms = '0'+ms; }

if (ts>0) { ds = ts; if (ts<10) { ds = '0'+ts; }} else { ds = '00'; }
dm=tm-1;
if (dm>0) { if (dm<10) { dm = '0'+dm; }} else { dm = '00'; }
dh=h-1;
if (dh>0) { if (dh<10) { dh = '0'+dh; }} else { dh = '00'; }

readout = dh + ':' + dm + ':' + ds + '.' + ms;
if (show==true) { document.clockform.clock.value = readout; }

clocktimer = setTimeout("startTIME()",1);
}

function findTIME() {
if (init==0) {
dateObj = new Date();
startTIME();
init=1;
} else {
if(show==true) {
show=false;
} else {
show=true;
}
}
}
</script>
<form name="clockform">
<input name="clock" style="text-align:center; width:174px; height:35px; font-size:24; font-weight:bold" value="00:00:00.00" /> <input name="starter" onclick="findTIME()" style="width:97px; font-weight:bold" type="button" value="Start/Stop" /> <input name="clearer" onclick="clearALL()" type="button" value="Reset" /> </form>
<hr />
<h2>