function relojDigital(){
var fecha = new Date(),
hor = fecha.getHours(),
min = fecha.getMinutes(),
seg = fecha.getSeconds(),
dia = fecha.getDay();
var lista = document.getElementsByTagName('span'),
reloj = document.getElementById('reloj');
if(seg < 10){seg = '0'+seg;}
if(min < 10){min = '0'+min;}
if(hor > 12){
hor = hor - 12, hor = '0'+hor;
document.getElementById('PM').className = 'active';
}
lista[dia].className = 'active';
reloj.textContent = hor+':'+min+':'+seg;
}
relojDigital();
setInterval(relojDigital,1000);
<div id="MiReloj">
<p id="Dias">
<span>Dom</span>
<span>Lun</span>
<span>Mar</span>
<span>Mie</span>
<span>Jue</span>
<span>Vie</span>
<span>Sab</span>
</p>
<p id="AM">AM</p>
<p id="reloj"></p>
<p id="PM">PM</p>
</div>
* {
font-family: 'Poiret One', cursive;
margin:0;
padding:0;
}
div#MiReloj {
background: gray;
overflow: hidden;
color: rgba(224, 224, 224, 0.5);
display:inline-block;
text-shadow: 1px 1px 0px #656565;
margin: 10px;
padding: 5px;
border-radius: 5px;
box-shadow: inset 0px 0px 10px;
}
p#AM, p#reloj, p#PM {
float: left;
display: inline-block;
padding: 5px 10px;
}
p#PM {
float: right;
}
p#Dias span {
display: inline-block;
padding: 5px 10px;
}
p#AM, #PM {
position: relative;
bottom: -30px;
width: 65px;
text-align: center;
}
p#reloj {
font-size: 40px;
width: 100px;
color: #e0e0e0;
}
.active {
color: #e0e0e0;
font-weight:700;
}
External resources loaded into this fiddle: