I’ve been using the Javascript Date API for a calendar application, and I noticed that for every month between 3 and 9 (April and November) the methods Date.getDay() and Date.getUTCDay() return different results.

For each of these months, Date.getUTCDay() will consistently return the same as Date.getDay() - 1.

I eventually noticed this only happened when I built the dates with the default constructor. Once I tried using the Date.UTC() method to build the dates, everything went as expected.

Which is to say:

Date date = null;
for (var month = 0; month < 12; month++) {
  date = new Date(2014, month, 1);
  if (date.getUTCDay() !== date.getDay()) {
    console.log(month);
  }
}

Will output: 3, 4, 5, 6, 7, 8 and 9.

Date date = null;
for (var month = 0; month < 12; month++) {
  date = new Date(Date.UTC(2014, month, 1));
  if (date.getUTCDay() !== date.getDay()) {
    console.log(month);
  }
}

Will not output anything.

I’m assuming it has something to do with DST, but I’d still like to have some deeper understading on what is happening and why.

Aside: I’m also assuming this is not something specifically related to Javascript, which is why I tagged the question with language-agnostic. Feel free to re-tag as appropriate.

Edit: Completely forgot to update the title after writing the question. I apologize.