I suggest this little hack to prevent your problematic case:
function convert_time(duration) {
var a = duration.match(/\d+/g);
if (duration.indexOf('M') >= 0 && duration.indexOf('H') == -1 && duration.indexOf('S') == -1) {
a = [0, a[0], 0];
}
if (duration.indexOf('H') >= 0 && duration.indexOf('M') == -1) {
a = [a[0], 0, a[1]];
}
if (duration.indexOf('H') >= 0 && duration.indexOf('M') == -1 && duration.indexOf('S') == -1) {
a = [a[0], 0, 0];
}
duration = 0;
if (a.length == 3) {
duration = duration + parseInt(a[0]) * 3600;
duration = duration + parseInt(a[1]) * 60;
duration = duration + parseInt(a[2]);
}
if (a.length == 2) {
duration = duration + parseInt(a[0]) * 60;
duration = duration + parseInt(a[1]);
}
if (a.length == 1) {
duration = duration + parseInt(a[0]);
}
return duration
}