Minute count/timestamp conversation

var helpers = {
    mc: function(ts) {
        var s = 1;
        if (["+","-"].indexOf(ts.charAt(0)) != -1) {
            if (ts.charAt(0) == "-") s = -1;
            ts = ts.substring(1);
        }
        var h = parseInt(ts.substring(0, 2));
        var m = parseInt(ts.substring(3, 5));
        return (h >= 0 && m >= 0 && m < 60) ? s * (60 * h + m) : NaN;
    },
    ts: function(M) {
        var d = function(i) { return (i < 10 ? "0" : "") + i };
        M = parseInt(M);
        if (!isNaN(M)) {
            var mi = M < 0;
            M = Math.abs(M);
            var m = parseInt(M / 60);
            var s = M % 60;
            return (mi ? "-" : "") + d(m) + ":" + d(s);
        } else return undefined;
    }
};