Convert to milliseconds when adding time in JavaScript

To keep time uniform, JavaScript represents it as the number of milliseconds since midnight January 1st, 1970 GMT. As a result, whenever you want to mathematically manipulate a time, you'll need to convert everything to milliseconds.

For example, the following code illustrates how to add eight hours to the current time:

var dtHere = new Date();

var oneDayFromNow = dtHere.getTime() + (8 * 60 * 60 * 1000);

dtHere.setTime(oneDayFromNow);