Hola paisano.
Añade las líneas que están en negrita.
<script language="javaScript">function calct(form){
var today=new Date();
var m1=0;
var m2=today.getMinutes();
form.lgmt.value=form.loclt.options[form.loclt.selectedIndex].value;
form.fgmt.value=form.fort.options[form.fort.selectedIndex].value;
form.tdif.value=form.fgmt.value-form.lgmt.value;
m1=parseInt(today.getHours())+parseInt(form.tdif.v alue);
if (10>m2){m2="0"+m2};
form.ltime.value=today.getHours()+":"+m2;
if (m1>=24) m1-=24
if (m1<0) m1+=24
form.ftime.value=m1+":"+m2;
}</script>