Question

Could someone explain to me what this returning number means? and how it is derived to that?

console.log(Date.now() - 24 * 60 * 60 * 1000);

If I wanted to use the above formula to display the next 15minutes and not 24 hours? how would I alter it?

Answer 1

Date.now() returns:

the number of milliseconds elapsed since 1 January 1970 00:00:00 UTC.

24 * 60 * 60 * 1000 in milliseconds represents 24 hours^*. So you basically get a timestamp 24 hours in the past from now. Notice that due to DST this doesn’t necessarily compute a timestamp one day in the past. It’s 24 hours in the past.

Also to get some meaningful output you should wrap resulting number in Date:

console.log(new Date(Date.now() - 24 * 60 * 60 * 1000));

Finally Date.now() can be replaced with new Date() when using in arithmetic expression.

^{* – 24 (hours) times 60 (minutes in hour) times 60 (seconds in minute) times 1000 milliseconds in second}.