The Hidden Complexities of the getDay Method in JavaScript

Ever wondered why your JavaScript code sometimes returns the wrong day of the week? The getDay method can be surprisingly tricky, especially when working with date strings and timezones. Let’s dig into the hidden complexities and learn how to get accurate results.

In JavaScript, we often use Date objects to represent specific points in time. Here, I wanted to determine the day of the week for a date string in the “YYYY-MM-DD” format.

First, let’s clarify:

The getDay method of the Date object returns the day of the week as a number from 0 (Sunday) to 6 (Saturday). It does not return the day of the month.

If you need the day of the month, use the getDate method instead. This returns a number from 1 to 31, representing the day of the month for the Date object.

Initially, my function to get the weekday looked like this:

function getWeekDay(dateString) {
    const date = new Date(dateString);
    const dayOfWeek = date.getDay();
    const weekDays = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
    return weekDays[dayOfWeek];
}

However, during testing, I noticed the function consistently offset the day by 1. After some research, I discovered that ECMAScript Date objects are based on a time value offset from 1970-01-01T00:00:00Z (UTC). This means the value returned by getDay can vary depending on the host’s timezone offset.

To address this, you can specify the timezone offset when creating a Date object using the constructor’s optional fourth argument. This argument represents the number of minutes the timezone is offset from UTC. For example, to create a Date object for “January 1st, 2022” in a timezone 2 hours behind UTC (120 minutes), use:

const date = new Date(2022, 0, 1, 0, -120);

By specifying the timezone offset, you ensure the Date object is created with the correct time value for your desired timezone, and getDay should return the correct weekday.

However, this approach isn’t ideal, since you must determine the timezone offset to create the Date object accurately. A simpler alternative is to use the new Date constructor without specifying the timezone offset:

const date = new Date(year, month, day);

Then, use getDay on the new Date object to get the day of the week. This method is easier to understand and maintain, as it doesn’t require knowledge of the timezone offset.

Keep in mind, the month argument for the new Date constructor is zero-indexed (0 for January, 1 for February, etc.). This can be confusing if you’re not familiar with JavaScript’s convention, so remember to subtract one from the month value.

For example, to create a Date object for “January 1st, 2022”:

const date = new Date(2022, 0, 1);

Here’s the final version of the code:

function getWeekDay(dateString) {
    // dateString is sanitized yyyy-mm-dd format string
    const parts = dateString.split("-");
    const year = parseInt(parts[0], 10);
    const month = parseInt(parts[1], 10) - 1;
    const day = parseInt(parts[2], 10);

    const date = new Date(year, month, day);

    const dayOfWeek = date.getDay();
    const weekDays = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
    return weekDays[dayOfWeek];
}