Need Help with JavaScript and time.

EDIT: Made some changes to the code.

Hi Andrew,

Had a go and came up with something that might help. It might not work for all time zones though so you'll need to test it.

User selects a date with date picker, it logs to the console an array containing all the dates/times, in 15 minute intervals, from 08:00am to 07:45am the next day.

I've added some comments to help explain my code and will try to explain it below, but I don't completely understand it myself yet!

Codepen & Codepen Full

var Times = [];

$("#datepicker").datepicker({ 
    // dateFormat is unix timestamp format - this is the primary format that we will work with.
    dateFormat: '@', 
    //  altFormat is the the format displayed to the user. Need to assign it to the #datepicker input with altField.
    altField: '#datepicker',
    altFormat: 'MM-dd-yy',
    onSelect: function(dateText) { 
    var selectedDate = dateText; 

    // selectedDate = date picked @ 12:00am. Dividing by 1000 converts the date from ms to epoch?  
    var dividedDate = selectedDate / 1000; 

    // Because dividedDate = date picked @ 12:00am, we need to add 8 hours (28800000ms / 1000 = 28880) to make it date picked @ 08:00am
    var startDate = dividedDate + 28800; 

        // There are 96 x 15min periods in 24 hours (24/96 = 0.25). We can go through each one with a loop. 
        for (var i = 0; i < 96; i++) {
           // first time round the loop counter = 0, second time = 900...
            var counter = i * 900;

           // newVal = date picked + 900...1800..2700 etc.
            newVal = startDate + counter;
            // Convert the value to a regular date. Need to * 1000 because we had previously / 1000
            var ToDate = new Date(newVal * 1000);

            // creates the array. Index 0 = 08/08/2015 @ 08:00, Index 95 = 09/08/2015 @ 07:45
            Times.push(ToDate);

        }
       // log the array to console
        console.log(Times);

    } // end onSelect
}); // end datePicker

Things to note:

Time zones really screw things up when it comes to dates. I am using the GMT timezone, this might not work for yours.

I am using unix time. If you've not heard of this before basically some guy started a clock on 00:00 Thursday, 1 January 1970. By counting the seconds since then we are able to work out dates/times.

I choose a starting date of 08/08/2015 (8th of August in my country). Then worked out what the unix/epoch time values are at 08:00, 08:15, 08:30, 08:45 and 09:00 (am). To do this I used the Epoch Converter

In my timezone, the values were as follows (if your using the converter above this value is the 'Epoch Timestamp'):

1439020800 (08/08/2015 08:00)

1439021700 (08/08/2015 08:15)

1439022600 (08/08/2015 08:30)

1439023500 (08/08/2015 08:45)

1439024400 (08/08/2015 09:00)

There is a difference of 900 between each value: 1439020800 (08:00) + 900 = 1439021700 (08:15)

According to google 15 minutes = 900000ms.

Doing 900000ms / 1000 = 900. - I think dividing by 1000 converts from unix time to epoch time?

So if we can get the value at 08/08/2015 08:00 (1439020800 in epoch time). Then we can just keep adding 900 to get a new value every 15 mins - 08/08/2015 08:15 (1439021700), 08/08/2015 08:30 (1439022600)... Stopping when we get to 09/08/2015 07:45.

When a user selects a date the date picker will return a value that is 1000 times larger than what we need (e.g 1439020800000). So we need to divide it by 1000 just as we did with the 900000ms. (You'll need to remove altField and altFormat to see this happening)

Hope this helps. Had fun doing it so thanks! Let me know what you think :)