Try this:
function converToMinutes(s) {
var c = s.split('.');
return parseInt(c[0]) * 60 + parseInt(c[1]);
}
function parseTime(s) {
return Math.floor(parseInt(s) / 60) + "." + parseInt(s) % 60
}
//var minutes = parseTime(EndTIme) - parseTime(StartTime);
var timeToSubtract = 40;
var startTime = converToMinutes('11.30');
var converted = parseTime(startTime - timeToSubtract);
alert(converted);