Do not concatenate or If

Multiple errors. Put the line that prints arrow before the write and use () after the getHours etc. So:

var tempo=new Date();
var hora=tempo.getHours();
var min=tempo.getMinutes();
var seg=tempo.getSeconds();

if(hora<10){
    hora="0" + tempo.getHours();
}
if(min<10){
    min="0" + tempo.getMinutes();
}
if (seg<10){
    seg="0" + tempo.getSeconds();
}

var impressao=hora + ":" + min + ":" + seg;

document.write(impressao);

But you can improve on that a lot. But I'll let you find out how;)

You are declaring the print variable before the if's, so it is picking up the values without the leading zeros. Put the statement after the if's and print.

You already have an accepted answer, but here is another, in my view cleaner and more DRY .

I usually use a simple function for this:

function pad(s) {
    return (s < 10) ? '0' + s : s;
}

This way you do not repeat code, it gets more organized.

Another tip for organizing is to [hora, min, seg].join(':'); instead of hora + ":" + min + ":" + seg; . Cleaner and uses less memory since concatenating strings generates a copy of each substring that is being concatenated. Note that this var impressao = ... line must be after you add 0 , so that it is included in this line, otherwise you have it now it ignores these last lines.

So your code could look like this:

function pad(s) {
    return (s < 10) ? '0' + s : s;
}

var tempo = new Date();
var hora = tempo.getHours();
var min = tempo.getMinutes();
var seg = tempo.getSeconds();

var impressao = [hora, min, seg].map(pad).join(':');
document.body.innerHTML = impressao;

jsFiddle: link