Percentage between two dates compared to today

Question

I am creating a UI which will contain Milestones and I will be using a progress bar to show this.

I am trying to come up with the percentage we are from the date the milestone is set to be completed vs today's date.

For example if I have a milestone called "Live to Site" that is set to happen on December 1, 2015, and I put the milestone in on Jan 1st, I want to determine the percentage we are from the start date to the end date.

Fiddle: http://jsfiddle.net/zz4c16fx/2/

var start = new Date(2015,0,1), // Jan 1, 2015 - The date we put this milestone in
    end = new Date(2015,7,24), // June 24, 2015 - The date the milestone is due
    today = new Date(), // April 23, 2015
    p = Math.round(100-((end - start) * 100 ) / today) + '%';

// Update the progress bar
$('.bar').css( "width", p ).after().append(p);

That was my attempt at it but either my math is wrong or I am thinking about this incorrectly. The number its returning is 99%. I feel that number should be lower seeing how we have 1.5 months left from today.

My end result is to show a progress bar that show how close we are to the completion date given the start, end and today's date.

Answer 1

Try this:

var start = new Date(2015, 0, 1), // Jan 1, 2015
    end = new Date(2015, 7, 24), // August 24, 2015
    today = new Date(), // April 23, 2015
    p = Math.round(((today - start) / (end - start)) * 100) + '%';
// Update the progress bar
$('.bar').css("width", p).after().append(p);

Demo: http://jsfiddle.net/zz4c16fx/6/