JavaScript works with the number of milliseconds since the epoch whereas most other languages work with the seconds. You could work with milliseconds but as soon as you pass a value to say PHP, the PHP native functions will probably fail. So to be sure I always use the seconds, not milliseconds.

unix time

//This will give you a Unix timestamp (in seconds):
var unix = Math.round(+new Date()/1000);

//This will give you the milliseconds since the epoch (not Unix timestamp):
var milliseconds = new Date().getTime();