I was playing around with the native Date object, and I realised that subtraction works (returns an integer representing the total number of miliseconds between the two dates),
You can simply convert current date into milliseconds, add your desired increment then convert it back to date time format and there you have your incremented date:
var d = new Date();
alert("Current DateTime: " + d);
var milliseconds = d.getTime(); //this will convert current date into milliseconds..
//Now youw want to progress the date by 3000ms.. simply add it to the current date time..
milliseconds += 3000;
d = new Date(milliseconds); //your new incremented date
alert("After 3000ms: " + d);
See the DEMO here