Adding 30 minutes to Date causes it to go back 30 minutes

Question

I have a Javascript Date object equal to 00:30 and when doing:

date.setMinutes(date.getMinutes() + 30);

causes the date object to equal 00:00.

Does anyone know why this is happening?

Here is where the code is being used:

for (var i = openTime; i <= closeTime; i.setMinutes(i.getMinutes() + timeIncrement)) {
  var time = i.getHours() + (i.getHours() == 0 ? '0' : '') + ':' + i.getMinutes() + (i.getMinutes() == 3 || i.getMinutes() == 0 ? '0' : '');

  $(timeClientId).append($('<option />').val(time).text(time));
} 

The above script creates a list of times available from 10:00am all the way to 02:00am the next day.

It runs fine until it reaches midnight 00:00 after many successful iterations.

Can anyone help?

Thanks!

ANSWER/SOLUTION:

This problem was due to a daylight saving issue, so this Saturday the clocks go forward. For some odd reason when adding 30 minutes to 12:30 it reset back to 12:00 using .setMinutes(). This kept it in an endless loop. The solution was to add minutes using i.setTime(i.getTime() + timeIncrement * 60 * 1000) This sorted the issue.

Cheers for all your answers!

Answer 1

You are only setting the minutes. So of course 30 minutes + 30 minutes on the clock equals 60 minutes, i.e. 0 minutes.

Use this clever method (it's clever because it works with all rollovers!):

function addMinutes(inDate, inMinutes)
{
    var newdate = new Date();
    newdate.setTime(inDate.getTime() + inMinutes * 60000);
    return newdate;
}

var date = new Date();

alert(addMinutes(date,-30));​

Answer 2

How are you initialising your date? setMinutes seems to work as expected so perhaps there is an error in your initial value. See below for my quick n dirty test.

var date = new Date(0);
document.write(date);
document.write("<br>");
date.setMinutes(30);
document.write(date);
document.write("<br>");
date.setMinutes(date.getMinutes() + 30);
document.write(date);
document.write("<br>");

Outputs:

Thu Jan 1 00:00:00 UTC 1970
Thu Jan 1 00:30:00 UTC 1970
Thu Jan 1 01:00:00 UTC 1970