How Many Seconds Between Two Dates?

I'm taking YYYY & ZZZZ to mean integer values which mean the year, MM & NN to mean integer values meaning the month of the year and DD & EE as integer values meaning the day of the month.

var t1 = new Date(YYYY, MM, DD, 0, 0, 0, 0);
var t2 = new Date(ZZZZ, NN, EE, 0, 0, 0, 0);
var dif = t1.getTime() - t2.getTime();

var Seconds_from_T1_to_T2 = dif / 1000;
var Seconds_Between_Dates = Math.abs(Seconds_from_T1_to_T2);

A handy source for future reference is the MDN site

Alternatively, if your dates come in a format javascript can parse

var dif = Date.parse(MM + " " + DD + ", " + YYYY) - Date.parse(NN + " " + EE + ", " + ZZZZ);

and then you can use that value as the difference in milliseconds (dif in both my examples has the same meaning)

Just subtract:

var a = new Date();
alert("Wait a few seconds, then click OK");

var b = new Date();
var difference = (b - a) / 1000;

alert("You waited: " + difference + " seconds");

You can do it simply.

var secondBetweenTwoDate = Math.abs((new Date().getTime() - oldDate.getTime()) / 1000);