javascript date minus seconds

// datenow minus 20 seconds
new Date(Date.now() - 20000);

var d;
d = new Date('2014-01-01 10:11:55');
alert(d.getMinutes() + ':' + d.getSeconds()); //11:55
d.setSeconds(d.getSeconds() + 10);
alert(d.getMinutes() + ':0' + d.getSeconds()); //12:05